As null hypothesis is associated with type I error, the alternative hypothesis is associated with type II error, when we are not able to reject the null hypothesis. Conventionally, the power is set at 0. Parallel RCT design is most commonly used, which means all participants are randomized to two the most common or more arms of different interventions treated concurrently. Based on the nature of relevant amount, superiority design contains statistical superiority trials and clinical superiority trials.
The objective of this design is to ascertain that the new treatment and standard treatment are equally effective. The null hypothesis of that is: Both two treatments differ by a clinically relevant amount. Non-inferiority trials are conducted to show that the new treatment is as effective but need not superior when compared to the standard treatment.
The corresponding null hypothesis is: The new treatment is inferior to the control treatment by a clinically relevant amount. One-sided test is performed in both superiority and non-inferiority trials, and two-sided test is used in equivalence trials. The hypothesis testing of different design is summarized in Table 1. Assuming RCT has two comparison groups and both groups have the same size of subjects; sample size calculation depends on the type of primary outcome measures.
Problem: The research question is whether there is a difference in the efficacy of mirtazapine new drug and sertraline standard drug for the treatment of resistant depression in 6 -week treatment duration. Problem: The research question is whether there is a difference in the efficacy of ACE II antagonist new drug and ACE inhibitor standard drug for the treatment of primary hypertension.
Change of sitting diastolic blood pressure SDBP, mmHg is the primary measurement, compared to baseline. Indeed, the steps for calculating sample size mirror the steps required for designing a RCT.
Firstly, the researcher should specify the null and alternative hypotheses, along with the type I error rate and the power 1- type II error rate.
Secondly, the researcher can gather the data of relevant parameters of interest but sometimes a pilot study may be required. Thirdly, the sample size can be estimated based on several reasonable parameters. In fact the key point which readers need to know is about the choice of null and alternative hypothesis, which should be adjusted according the study objective.
This parameter has clinical significance, which should be cautiously determined and it must be reasonable. This paper gives simple introduction of principals and methods of sample size calculation. Comment Post Cancel. Thank you for your reply and advise Phil. I haven't started the trial yet. As my outcome will be in terms of percentage of patients with complete relief.
While I was looking on stata, I could see sample size estimation command for comparing 2 independent proportions but could not sort out how can I compute sample size if I have 3 groups instead of 2 groups. I will surely go through FAQ to see if I can find something similar. Thanks again Regards Neha. Error 2: If analysis concludes that paracetamol is equal to placebo, we accept H 0. Knowing that paracetamol is better than placebo H 1 we are making an error here by accepting H 0.
This is called as Type II error. In above case if analysis concludes that paracetamol is better than placebo, we reject H 0 , which would be correct decision. In CT, different statistical study designs are available to achieve objectives. Typical designs that may be employed are parallel group design, crossover design etc. For sample size estimation study design should be explicitly defined in the objective of the trial. A cross over design will have different approach and formula for sample size estimation as compared to parallel group design.
This is another critical parameter needed for sample size estimation, which describes aim of a CT. The aim can be equality, non-inferiority, superiority or equivalence. Equality and equivalence trials are two-sided trials where as non-inferiority and superiority trials are one-sided trials. Superiority or non-inferiority trials can be conducted only if there is prior information available about the test drug on a specific end point.
The sample size calculation depends on primary end point of a CT. The description of primary study end point should cover whether it is discrete or continuous or time-to-event. Sample size is estimated differently for each of these end points. Sample size is adjusted if primary end point involves multiple comparision. The information about expected response is usually obtained from previous trials done on the test drug.
If this information is not available, it could be obtained from previous published literature. The important information required is:. This is one of most critical and one of most challenging parameters. The challenge here is to define a difference between test and reference which can be considered clinically meaningful.
To put it in plain English, this is the difference which will make your family doctor to give you new medication over and above the existing gold standard which he is prescribing for last 10 — 20 years.
This threshold figure is at times not easily available and should be decided based on clinical judgment. Type I error is inversely proportional to sample size. As per guideline. Type II error is directly proportional to sample size. The sample size estimation formula will provide number of evaluable subjects required for achieving desired statistical significance for a given hypothesis. However in practice we may need to enroll more subjects to account for potential dropouts. Study to find an association: A study to find an association determines if a variable, the dependent variable, is affected by another, the independent variable.
For instance, a study to determine whether blood pressure is affected by salt intake. Measurement: The outcome of the study is a continuous measurement. Time to an Event: The outcome of the study is a time, such as the time to death, or relapse.
Some patients will not have been observed to relapse. These observations are said to be censored.
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