If previous studies showed a reduction of 0.71% in Hba1c (SD = 0.55) in inetrvention group and 0.25% (SD=0.47) in control group what will be the required sample size for this study to achieve 0.05 level of signficance and 95% statistical power
There is no fixed sample size. It depends upon several factors like precision of the findings, expenditure involved, time available for data collection, the characteristics of the population under study, the seriousness of the findings to be obtained etc. Since the concerned population under study i a clinical trial in infinite, sample size is to be sufficiently large. The process is like this:
Select a sample and analyze the data.
Increase the sample size and analyze the data.
Compare the findings of the two analysis.
Continue the process.
When the findings of successive two analysis become identical stop increasing sample size.
Unfortunately I don't think it is possible to answer your question based on the information you have provided. You need to calculate the effect size of the previous intervention and also provide the ratio of the experimental and control group sizes. You should then be able to calculate the required sample size, assuming that you are using an independent samples t-test. For example, you can use http://biomath.info/power/ttestnoninf.htm.
how can I calculate effect size of previous intervention ? I mentioned already educational intervention in previous studies lower HbA1c by 0.71% in intervention group and 0.25 in control group
The simplest and most straightforward effect size measure is the difference between two means. But the limitation of this measure as an effect size is accuracy. It’s just hard to evaluate. Instead, the Standardized effect size which is obtained by dividing the difference by either the standard deviation of the control group (giving you a Glass’s Δ) or by the pooled standard deviation of both groups (giving you Cohen’s d or Hedges’ g depending on the pooling equation used) can be used. Standardized effect sizes are designed for easier evaluation. They remove the units of measurement, so you don’t have to be familiar with the scaling of the variables.
I am not sure why you’re going for 95% statistical power, mostly it is 80% and sometimes it is 90%. As it is RCT it is unethical to just select a sample and analyze the data. Do a proper calculation prior to start.
Dear Kandasamy thank you for answer, but can you calculate me the required sample size if we assume stistical power of 90%? According to my calculations it will be 62 patients (31 in each group) is that number true?
I also like to say that I am not sure why you’re going for 95% statistical power, mostly it is 80% and sometimes it is 90%. As it is RCT it is unethical to just select a sample and analyze the data.
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