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- For example, suppose there are randomized trials directly comparing provision of dietary advice by a dietitian which we refer to as intervention A with advice given by a doctor intervention B. Suppose there are also randomized trials comparing...
- A versus B and A versus C can be treated as two subgroups in the meta-analysis. The difference between the summary effects from the two subgroups gives an estimate for the indirect comparison. Most software will provide a P value for the statistical...
- Similarly, any other effect modifiers should not differ between AB and AC studies. Transitivity requires all competing interventions of a systematic review to be jointly randomizable. That is, we can imagine all interventions being compared simultaneously in a single multi-arm randomized trial. Researchers undertaking indirect comparisons should assess whether such differences are sufficiently large to induce intransitivity. In principle, transitivity can be evaluated by comparing the distribution of effect modifiers across the different comparisons Salanti , Cipriani et al , Jansen and Naci Imbalanced distributions would threaten the plausibility of the transitivity assumption and thus the validity of indirect comparison. In practice, however, this requires that the effect modifiers are known and have been measured. There are also some statistical options for assessing whether the transitive relationship holds in some circumstances, which we discuss in Section Extended guidance on considerations of potential effect modifiers is provided in discussions of heterogeneity in Chapter 10, Section For example, we may believe that age is a potential effect modifier so that the effect of an intervention differs between younger and older populations.
- If the average age in A versus B randomized trials is substantially older or younger than in A versus C randomized trials, transitivity may be implausible, and an indirect comparison B versus C may be invalid. Suppose a single effect modifier is severity of disease e. The top row depicts a situation in which all patients in all trials have moderate severity. There are AB studies and AC studies in this population. Estimation of BC is valid here because there is no difference in the effect modifier.
- The second row depicts a similar situation in a second population of patients who all have severe disease. A valid indirect estimate of B versus C for this population can also be made. In the third row we depict a situation in which all AB trials are conducted only in moderately obese populations and all AC trials are conducted only in severely obese populations. In this situation, the distribution of effect modifiers is different in the two direct comparisons, so the indirect effect based on this row is invalid due to intransitivity. Intransitivity would probably still invalidate the indirect estimate B versus C if severity is an important effect modifier. The shaded boxes represent the treatment effect estimates from each source of evidence striped box for A versus B and checked box for A versus C.
- In the first row, randomized trials of A versus B and of A versus C are all conducted in moderately obese populations; in the second row randomized trials are all conducted in severely obese populations. In both of these the indirect comparisons of the treatment effect estimates would be valid. In the last row, the A versus B and A versus C randomized trials are conducted in different populations. As severity is an effect modifier, the indirect comparison based on these would not be valid Jansen et al Reproduced with permission of Elsevier If the key assumption of transitivity is considered reasonable, direct and indirect estimates should be considered jointly.
- When both direct and indirect intervention effects are available for a particular comparison, these can be synthesized into a single effect estimate. This summary effect is sometimes called a combined or mixed estimate of the intervention effect. We will use the former term in this chapter. A combined estimate can be computed as an inverse variance weighted average see Chapter 10, Section Since combined estimates incorporate indirect comparisons, they rely on the transitivity assumption.
- Because network meta-analysis could be used to estimate the relative ranking of the included interventions Salanti et al , Chaimani et al , reviews that aim to rank the competing interventions should specify this in their objectives Chaimani et al Review authors should consider obtaining an estimate of relative ranking as a secondary objective to supplement the relative effects. An extended discussion on the relative ranking of interventions is provided in Section A driving principle is that any eligible participant should be eligible for randomization to any included intervention Salanti , Jansen and Naci Review authors should select their target population with this consideration in mind.
- Particular care is needed in the definition of the eligible interventions, as discussed in Chaimani and colleagues Chaimani et al For example, suppose a systematic review aims to compare four chemotherapy regimens for a specific cancer. The remaining two regimens B and C are appropriate for stage I patients exclusively. The four interventions forming the network are unlikely to satisfy the transitivity assumption because regimen D is not given to the same patient population as regimens B and C. Thus, a four-arm randomized trial comparing all interventions A, B, C and D simultaneously is not a reasonable study to conduct. These are the options among which patients and health professionals would be choosing in practice with respect to the outcomes under investigation.
- In selecting which competing interventions to include in the decision set, review authors should ensure that the transitivity assumption is likely to hold see also Section The ability of network meta-analysis to incorporate indirect evidence means that inclusion of interventions that are not of direct interest to the review authors might provide additional information in the network. For example, placebo is often included in network meta-analysis even though it is not a reasonable treatment option, because many studies have compared active interventions against placebo. In such cases, excluding placebo would result in ignoring a considerable amount of indirect evidence.
- Similar considerations apply to historical or legacy interventions. We use the term supplementary set to refer to interventions, such as placebo, that are included in the network meta-analysis for the purpose of improving inference among interventions in the decision set. The full set of interventions, the decision set plus the supplementary set, has been called in the literature the synthesis comparator set Ades et al , Caldwell et al When review authors decide to include a supplementary set of interventions in a network, they need to be cautious regarding the plausibility of the transitivity assumption.
- In general, broadening the network challenges the transitivity assumption. Thus, supplementary interventions should be added when their value outweighs the risk of violating the transitivity assumption. The addition of supplementary interventions in the analysis might be considered more valuable for sparse networks that include only a few trials per comparison. In these networks the benefit of improving the precision of estimates by incorporating supplementary indirect evidence may be quite important. Inevitably it will require some judgement, and the robustness of decisions can be evaluated in sensitivity analyses and discussed in the review. The appropriateness of merging, for example, different doses of the same drug or different drugs within a class depends to a large extent on the research question.
- Lumping and splitting the variants of the competing interventions might be interesting to both review authors and evidence users; in such a case this should be stated clearly in the objectives of the review and the potential for intransitivity should be evaluated in every network. A decision on how the nodes of an expanded network could be merged is not always straightforward and researchers should act based on predefined criteria where possible. These criteria should be formed in such a way that maximizes similarity of the interventions within a node and minimizes similarity across nodes. We sought advice from experts in smoking cessation on the key dimensions that would influence the effectiveness of smoking cessation programmes.
- Through this process, two dimensions for evaluating interventions were identified. The first dimension was related to whether the intervention offered generic advice or tailored its feedback to information provided by the user in some way.
- You should spend about 40 minutes on this task. Write about the following topic: In many countries, people are concerned about the number of children who are overweight. What do you think are the causes of this? What solutions can you suggest? Write at least words. Children become fat because of modernization and technology advancement. This essay will discuss the problems of obesity in young people and the responsibilities of the government, society, and parents in this regard. The major causes giving rise to this problem are the change in our lifestyle due to the remarkable advances in technology, which has a considerable impact on our daily life and health. Children consume junk foods at any time, and it makes matters worse.
- These foods, which are easily available, often contain ingredients that are high in calorie, cholesterol, and fat, while lacking necessary vitamins, minerals, and proteins. These factors are taking a toll on their health. In addition, teenagers spend most of the on their computers rather than playing outside. As a result, teenagers are lacking exercise and becoming overweight. In my opinion, teachers, parents, and the government as a whole should consider this issue and take the necessary steps. For instance, parents should keep an eye on the eating habits of their children and ensure that they eat healthy food and reduce their intake of junk food. The government could limit junk food advertising to prevent children from falling prey to bad eating habits.
- Band 8. This is due to a combination of poor diet and lack of exercise. Children tend to be attracted to food that is high in sugar and fat, such as various forms of fast food. They also tend to be ignorant of what constitutes a healthy diet. Although parents may try to guide their children, they cannot be present always. As a result, children are influenced by other factors, such as advertising by fast-food companies. On top of that, children are much less active than they used to be. While in the past young people took part in activities that burned a considerable amount of energy, children today spend their time indoors, sitting at computers or playing video games. They have no opportunity to burn the calories they are taking in, leading to obesity. There are a number of steps that need to be taken to solve this problem.
- The first is to educate children about nutrition. Schools should organize compulsory lessons to explain the main elements of a healthy diet and teach the pupil how to prepare simple but healthy meals. Children also need to be encouraged to participate in sports. Again, schools have a large role to play in this, by offering a wide range of outdoor activities. Investment in local sports clubs by the government would also help in alleviating the problem. According to the medical experts, the situation is getting worse. I feel there are a number of reasons for this. Some people blame the fact that we are surrounded by shops selling unhealthy, fatty foods such as chips and fried chicken, at low prices. This has created a generation of adults who never cooked a meal for themselves. If there were fewer of these restaurants, then children would not be tempted to buy take-away food. There is another argument that blames the parents for allowing their children to become overweight.
- I tend to agree with this view, because good eating habits begin early in life, long before children start to visit fast food outlets. If children are given chips, ice creams and chocolate rather than nourishing food or are always allowed to choose what they eat, they will go for sweet and salty foods every time, and this will carry on throughout their lives. If parents try to make their kids understand that fast foods are not good for their health and draw a restriction on how much their kids are allowed to eat then the problem can be solved partially.
- There is a third factor, however, that contributes to the situation. Children these days do very little exercise. They do not walk to school. When they get home, they sit in front of the television or their computers and play video games. Not only is this an unhealthy pastime, but it also gives them time to eat more junk food. They need to go out and play active games or sport.
- The two views discussed play equal roles in contributing to the problem, but I think we have to encourage young people to be more active, as well as steer them away from fast food outlets and bad eating habits. We need to have a balanced approach. Approximately words This model answer has been prepared by the site developer. However, please note that this is just one example out of many possible answers. Model Essay 04 : All around the world the living standards are becoming higher and almost similar to the western society.
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This change in everyday life also brings in, its benefits and drawbacks, like the rapid increase in the number of overweight people. A lot of people think the cause of such a problem is the exponentially growing number of fast food outlets in developed countries. Fast foods have also become the easiest and cheapest solution to have lunch all over the world. Consequently, millions of families prefer to eat fast foods as the food is appealing and the cost is reasonable. In conclusion, both fast foods and lack of parental control are causes of the increasing number of overweight children. While this issue is being attributed to the growing number fast food restaurants, I would argue that parents should take some responsibility as well. To begin with, the expansion of convenient food stores may not be the cause of overweight children.- Although some of the foods served in these restaurants may contain high saturated fat items, there are other factors that could also result in gaining weight as well. Firstly, if children consumed an excessive amount of unhealthy snacks, sugary soda drinks, or prepackaged foods, they could also become obese. Secondly, even if the family did visit the fast food outlets for meals, parents could still substitute unhealthy items with a healthier menu, such as having French fries replaced with vegetables or salad. In addition, parents certainly play a vital role in providing a healthy diet for their children. With the invention of the Internet, it is possible to obtain information about food ingredients, daily nutrient proportion needed in each food category and healthy recipes within minutes.
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Therefore, learning how to prepare nutritious meals has never been easier. I believe supplying healthy snacks for children during school days and preparing a healthy balanced meal in the evening in routine could lead to a healthy body. However, it must not be forgotten that ensuring children have an adequate amount of physical exercise is also a critical factor to contribute to their good health. In conclusion, I believe that parents are the ones who play a vital role in upbringing their children in a healthy environment.Your IB Physics Course Book : Secondary: Oxford University Press
Keep up with your work. If you attend class regularly, keep up with readings, and take notes conscientiously, studying can be a relatively pain-free process. Make sure to review and expand upon class notes regularly throughout the semester. Consider developing a glossary or collection of note cards for vocabulary review in each class. Many students find that preparing for an individual class for minutes per day, five or six days per week, will leave them well-prepared at exam time. To assist students with organization at finals time, we have compiled a couple of time management tools that are included with this page.- Building off our previous entry, try studying for minutes per day for a week leading up to an exam. All-nighters simply don't work for most people, and students experience declining returns on their efforts when they attempt to study for four and five hours straight. Complete a mock test. So many social science, natural science, and foreign language text books contain hundreds of questions at the end of chapters that never get answered. Why not set aside an hour, and try to answer these questions on paper without using your notes? You may also combat pre-test jitters by demonstrating to yourself what you know. For the humanities, try answering a couple of potential essay questions on a timed, closed book basis and see how you do.
- Another simple way to conduct a mock test is to ask a friend or classmate to give you an oral quiz based on concepts in the textbook or in either of your notes. Do not multi-task while studying. Set aside time to study in advance and then follow through. If you have outstanding questions, go see your professor or tutor at least three days before the exam. Think about what written questions might be on the exam; Outline each potential essay as a form of pretesting and practice. Find a group of dedicated students with whom to study. A group study session is an ideal time to review and compare notes, ask each other questions, explain ideas to one another, discuss the upcoming exam and difficult concepts, and, when appropriate, delegate study tasks. Do set an agenda and a specific time frame for your group study session, so that your work together doesn't veer off-topic. Keep your ears open in class. Your professor will sometimes come right out and tell you about the exam or present study strategies.
- You need to be in class every day to receive such help. This is particularly true as tests and final exams approach. Use review sheets thoroughly. Review your class notes every day. Add keywords, summaries, idea maps, graphs, charts, discussion points, and questions where applicable. Take the time to organize lecture notes after class, adding key examples from labs and course readings. Take notes on the course readings. You should also review these notes on a regular basis. Again, create visual enhancements when possible e. Use both your course notebook and the text's margins to record valuable information.
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Please see our entries on reading for further information on this topic. Make sure to get plenty of sleep. You want to be as fresh as possible and able to fully engage your working memory when you take the exam. Find ways to apply materials from class. Think about how course topics relate to your personal interests, societal problems and controversies, issues raised in other classes, or different experiences in your life. Game-Day: Performing Well on the Exam 1. Eat a healthy breakfast. If music gets you going, go ahead and play something upbeat. Think of preparing like an athlete before a contest or a musician before a performance. When you first receive the exam, glance over the entire test before you start.- Create a plan of attack. Read the directions carefully. Remember that many questions at the college level have multiple queries or prompts. Write out a brief outline before beginning essay questions. Use the process of elimination on multiple-choice and matching questions. Also, for multiple choice questions, you may wish to cover the options first and try to answer the question on your own. As you prepare for multiple choice exams, make sure to be aware of context, relationships and positionality among concepts, and multiple definitions of terms. A deep understanding of vocabulary is a key to success on multiple-choice exams. Leave the most time-consuming problems for the end, especially those with low point values. Focus on the question at hand. If you complete the test one step at a time, you are much less likely to find it to be overwhelming. If you are stuck on a question, bypass it.
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Mark the question off, so you can return to it at the end of the exam. Show as much work as possible. This is particularly important for math exams. Make sure that you're answering each part of the question. If you have time at the end of the exam, go back and proofread your work and look over multiple-choice questions again. Check to see that you have answered every question before you turn in the exam. But remember, your first answer is usually your best answer. Be extremely cautious about changing answers later on. Some people benefit from conducting a memory dump when they first receive a test. That is, they jot down a comprehensive list of concepts, formulas, vocabulary, and details at the beginning and revisit these ideas as they're progressing through the test.- See if there is a way to draw a picture or otherwise create a visual description of the question you are trying to answer. Strive to include course terms and concepts in written responses correctly, of course. Post-Game: Reviewing Your Performance 1. If there was a part of the exam on which you struggled, go see your professor. Hold onto your notebooks. The same rule goes for many of your books. Take a moment to review your test preparation strategies. Take account of what worked and what needs improvement. In particular, take a moment to gauge whether your study group was helpful. If you feel like your test-preparation strategies need work, go see your professor or the Academic Advising Office. Reward yourself.
- Topic Electromagnetic induction HL See the guide for this topic. This phenomenon is called electromagnetic induction. Magnetic flux and magnetic flux linkage The magnetic flux through a surface measures the component of the magnetic field passing through the surface and is proportional to the number of magnetic field lines which intersect the surface. The magnetic flux linkage of a coil measures the component of the magnetic field passing through the coil. A coil of wire is driven to rotate by an external force. As the coil rotates, the magnetic flux linkage passing through the coil changes. If the rotation is at constant speed, the induced emf is sinusoidal recurring oscillation. Increasing the speed of rotation increases both the frequency and the magnitude of the induced emf. Average power and root mean square rms values of current and voltage The average power produced or dissipated from an alternating current cannot be computed directly using the peak values of voltage or current.
- The quoted values for alternating voltage and current refer to their rms values rather than their peak values. For an example, an ac socket in Europe is rated at V rms. Transformers A transformer is a device which can be used to transfer electrical energy from one ac circuit to another at a different voltage. Transformers which increase the output voltage are called step up transformers while those which decrease the output voltage are called step down transformers. A transformer is essentially two coil coils of wire coupled together as shown below. Alternating input voltage causes constant change in the magnetic field around the primary coil. This causes a constant change in the magnetic flux linkage in the secondary coil. The ratio between the induced input voltage rms and the output voltage rms is equal to the ratio between the number of turns in the corresponding coil.
- In other words, its power input is equal to power output. Half-wave and full-wave rectification Diodes are also known as rectifiers. They can be used to change alternating current ac to direct current dc through a process called rectification. Half-wave rectification The final dc waveform on the screen is the positive half only of the original ac waveform.
- In half-wave rectification, the negative part of the current is prevented from passing. Full-wave rectification The four diodes connected as shown in the diagram above form a full-wave rectifier. Through this arrangement, the positive half of each cycle is allowed to pass through while the negative half of each cycle is reversed. This diode configuration is commonly known as a diode bridge. A capacitor is made up of two metallic plates with a dielectric material in between the plates. When a voltage is applied over the two plates, an electric field is created with positive charge collecting in one plate and negative charge on the other. The capacitance of a capacitor can be correlated to the area of the plates A and the distance of separation between the plates d by FYI The potential energy stored in a capacitor is given by where E is the potential energy, Q is charge, V is voltage, and C is capacitance.
- Dielectric materials Dielectric materials are insulators, such as glass, plastics, distilled water, dry air, or even a vacuum. Dielectric materials keep the conducting plates in contact, allowing for smaller plate separation d , and higher capacitance C. Capacitors in series and parallel Resistor-capacitor RC series circuits An RC circuit is one where you have a capacitor and resistor in the same circuit. The charging and discharging of an RC circuit works like this: Assuming that C is fully discharged and that the switch is open. When the switch is closed in position 1, the battery is connected across the capacitor. Current flows and the potential difference across the capacitor begins to rise but as more and more charge build up on the capacitor plates, the current and the rate of rise of potential difference both fall until no further current flows and the potential difference across the capacitor equals that of the supply voltage V0.
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The capacitor is now fully charged. When the switch is then closed at position 2, a large current flows and the potential difference across the capacitor drops. As charge flows from one plate to the other through the resistor, the charge is neutralized. The current and the rate of decreasing potential difference both fall. When the charge on the plates reach zero, the initial condition, the current and potential difference are also zero. The capacitor is now fully discharged. Time constant The RC time constant specifies the time required to charge the capacitor, through a resistor, by approximately Share this:.
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