2 Sample t (sample size)
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 This topic has 2 replies, 3 voices, and was last updated 12 years, 1 month ago by StuW.

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November 3, 2009 at 2:21 pm #25806
MatthewGParticipant@MatthewG Include @MatthewG in your post and this person will
be notified via email.I’m trying to calculate a required sample size, but have a feeling I don’t have enough information.
Situation: Two Randomized Groups: 1 no intervention, 1 with intervention. If there is a 10% or greater increase in size, the intervention has made a difference.
I believe that is a 2Sample t Test (1 tailed). I assume Power = .8, but since there hasn’t been sampling, I don’t know SD.
If someone has a moment, can you let me know what I’m missing… it just isn’t computing (literally).
Cheers,
M
0November 4, 2009 at 1:51 pm #62504
DarshanParticipant@Darshan Include @Darshan in your post and this person will
be notified via email.Hope this helps …
http://www.dimensionresearch.com/resources/calculators/sample_size.html0November 4, 2009 at 11:46 pm #62505It is somewhat difficult from your description to tell exactly what test is being conducted. The calculator provided in the reply given is useful if you are measuring a percentage, but I’m not sure that is what you are measuring. Some questions that you need to answer to process this correctly:
– Are the units “paired” or independent samples? In other words, if the measurements are made on “like” units, then you can make the comparisons within each unit and have a single group test on the differences instead of two independent samples. This impacts the sample size calculations.
– You mention a 10% increase in size, but what is the base measurement? If it is a proportion, then use a sample size calculator based on 1 or 2sample proportions. If it is a measurement, then the test should be done using the estimate of a 10% change relative to the sigma expected. For example, suppose the base measurement is 100, so you are looking for a 10 unit increase. However, you need the estimate of variation in these measurements to apply a sample size calculator. The ratio of the 10 unit change to the process sigma is what you would need to enter in most SS calculators. You might need to do a few measurements to estimate this variation, otherwise your sample size estimation may be way off.
Hope this helps.0 
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