To measure the effectiveness of an intervention in social research, do we need to take same sample from Intervention and control or it can be measured even if we have less sample from control. In one of the study we have to increase sample in Intervention due as we did not had enough eligible for intervention but we kept the control sample same, will it affect. What is the best way to manage it?
Sekaran (2003) wrote:
"Roscoe (1975) proposes the following rules of thumb for determining sample size:
1. Sample sizes larger than 30 and less than 500 are appropriate for most research.
2. Where samples are to be broken into sub-samples; (male/females, juniors/seniors, etc.), a minimum sample size of 30 for each category is necessary.
3. In multivariate research (including multiple regression analyses), the sample size should be several times (preferably 10 times or more) as large as the number of variables in the study.
4. For simple experimental research with tight experimental controls (matched pairs, etc.), successful research is possible with samples as small as 10 to 20 in size."
Reference
Sekaran, U., 2003. Research methods for business: A skill building approach. John Wiley & Sons.
In you case, sample size calculation based on independent sample t-test, using G.power, with moderate effect size, power= .80, and at alpha= .05; you will need at least 32 participants per group. This means it is not a must to have the 2 groups exactly the same in sample size, but at least in each group to have 32. However, the variation should not be large between the groups.
Dear researcher,
I think the sample sizes should be the same to qualify the comparability across the two groups. Nonetheless, those ineligible cases should not be considered as a part of the sample.
Good luck
it can be less, there are many studies used different sample size in control and experimental groups. however, it should not be less than 30, and a final sample also depend on different factors like dropout, fail to attend certain sessions, researcher find few participants unsuitable for the study, etc.
- shahnawaz
Dear Abhishek,
According to Duflo, Glannerster and Kremer (2007) (available at http://economics.mit.edu/files/806) you can have a different sample size for your treatment and control group (see section 4.1). You can even compute the optimal sample size in each group depending based on the cost of the intervention and the cost of the data collection.
Best,
Quentin
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