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Thread: Sample size calculation

  1. #1
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    Sample size calculation

    Hey guys, I need some help calculating the minimum sample size needed for my upcoming trial. Iím doing higher degree research on eHealth, so I have little knowledge about statistics and sample size calculation. Was doing loads of research online and trying out online calculators but Iím still not too sure.


    So basically Iíve done an initial trial on a small sample of 24 participants, randomly allocated to two groups (13 in control group and 11 in intervention group), the aim is to see whether the use of an IT based intervention can improve health supplement adherence rate when compared to a control group using a pillbox. Below is the result from the initial trial:


    Control group- average baseline adherence rate before trial was 86%, and after trial the average adherence rate increased to 95%
    Intervention group- average baseline adherence rate before trial was 81%, and after trial the average adherence rate increased to 98%


    So what I would like to know is: what is the minimum number of participants I will need for each group (I want a 2:1 ratio, so more people in the intervention group). I need to know the very minimum number as I was having problem recruiting enough participants in the initial trial, and Iím currently starting the recruitment process for the upcoming full trial. I plan to recruit about 40 people in intervention and 20 in control, is this enough?


    Power=at least 80%, and alpha=0.05


    Iíve tried this online calculator, but unsure what to put in the anticipated incidence sections, or if it is the right calculator to use in my case: https://clincalc.com/stats/samplesize.aspx


    Any help will be appreciated.
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  2. #2
    MHF Contributor
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    Re: Sample size calculation

    Social sciences are substantially different from more concrete studies. For example, if you are looking for correlation, a mathematician or actuary might laugh at you if you managed only 90%. On the other hand, a psychologist might be DELIGHTED at only 40%. My implication is this, that your study design and intent should control your sample size at least as much as your Statistics I Calculator. You MUST judge between COST and VALUE. For example, if you are blowing up missiles at $20 million each, you'll want to do that as little as possible and include your computer simulations. If you are doing a mortality study, most folks would believe you have NOTHING until you have AT LEAST 60 deaths in your SMALLEST cell. The super basic rule for just about anything is to enforce at least 30 participants in your smallest group. That puts you at 90 participants. I'm going to guess that's too expensive. In some other words, your COST may dictate more than your Confidence or your Power. Note also the assumptions made by online calculators... Are you assuming NORMAL? Do you have that? Are you assuming EQUAL VARIANCE? Do you have that? Are you assuming SYMMETRIC? Do you have that? Go ahead and play with any online calculator you find. See if you get the same results. No matter how many numbers you get, you'll need to THINK about it. This is THE MOST DIFFICULT part of any such experiment - 1) DEFINING what you want and 2) Weighing what you want vs. what you can do.
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