calculating sample sizes for independent groups
Can someone give me the spreadsheet formulas for calculating a sample size
for independent samples? Thanks Ariel |
calculating sample sizes for independent groups
I'm sure someone can ... but you need to learn to state the requirements
clearly first. __________________________________________________ ______________________ "Ariel" wrote in message ... Can someone give me the spreadsheet formulas for calculating a sample size for independent samples? Thanks Ariel |
calculating sample sizes for independent groups
I had to look this up, its ages since did this.
It is based on the standard error of the mean. Say a sample of 100 items has a mean of 50 and a standard deviation of 20. The interval estimate with 95% confidence for the population mean is: =1.96*std_Dev/SQRT(sample_size) which gives four. The table below shows the figures and formulas. (I created Range names for the numbers to make it more clear. Select labels and figures and choose Insert, Names, Create - make sure the left labels checkbox is ticked and click OK sample size 100 100 mean 50 50 std Dev 20 20 Error of mean 3.92 =1.96*std_Dev/SQRT(sample_size) Mean + Err 53.92 =mean+b5 Mean - Err 46.08 =mean-b5 new N 384.16 =(std_Dev*1.96/2)^2 so to reduce the error of the mean by half we increase the sample size by: =(std_Dev*1.96/2)^2 of course you have to get you mean and stdev first or make an estimate of them first. Hope this helps Peter "Ariel" wrote: Can someone give me the spreadsheet formulas for calculating a sample size for independent samples? Thanks Ariel |
calculating sample sizes for independent groups
Hi Billy,
Thank you for your effort, but this calculation is not for two independent groups. Assume that the mean and SD of the first group is 0.50 and 0.224, respectively. We estimate that the second group will have a mean value of 0.045 (difference between the two groups is 10%). I can even throw in that we estimate that the second group's SD will be 0.212. What will the sample size needed to acheive this in the second group, assuming a power of 80% and an alpha of 0.05.? An online calculator gave me a sample size of 29,864 if the two SDs are known (using the z-test) and a sample size of 31,506 when only the first SD is known and the second estimated. Thanks for your help! "Billy Liddel" wrote: I had to look this up, its ages since did this. It is based on the standard error of the mean. Say a sample of 100 items has a mean of 50 and a standard deviation of 20. The interval estimate with 95% confidence for the population mean is: =1.96*std_Dev/SQRT(sample_size) which gives four. The table below shows the figures and formulas. (I created Range names for the numbers to make it more clear. Select labels and figures and choose Insert, Names, Create - make sure the left labels checkbox is ticked and click OK sample size 100 100 mean 50 50 std Dev 20 20 Error of mean 3.92 =1.96*std_Dev/SQRT(sample_size) Mean + Err 53.92 =mean+b5 Mean - Err 46.08 =mean-b5 new N 384.16 =(std_Dev*1.96/2)^2 so to reduce the error of the mean by half we increase the sample size by: =(std_Dev*1.96/2)^2 of course you have to get you mean and stdev first or make an estimate of them first. Hope this helps Peter "Ariel" wrote: Can someone give me the spreadsheet formulas for calculating a sample size for independent samples? Thanks Ariel |
calculating sample sizes for independent groups
Hi Ariel
I think you lost me somewhere there! "Ariel" wrote: Hi Billy, Thank you for your effort, but this calculation is not for two independent groups. Assume that the mean and SD of the first group is 0.50 and 0.224, respectively. We estimate that the second group will have a mean value of 0.045 (difference between the two groups is 10%). I can even throw in that we estimate that the second group's SD will be 0.212. What will the sample size needed to acheive this in the second group, assuming a power of 80% and an alpha of 0.05.? An online calculator gave me a sample size of 29,864 if the two SDs are known (using the z-test) and a sample size of 31,506 when only the first SD is known and the second estimated. Thanks for your help! "Billy Liddel" wrote: I had to look this up, its ages since did this. It is based on the standard error of the mean. Say a sample of 100 items has a mean of 50 and a standard deviation of 20. The interval estimate with 95% confidence for the population mean is: =1.96*std_Dev/SQRT(sample_size) which gives four. The table below shows the figures and formulas. (I created Range names for the numbers to make it more clear. Select labels and figures and choose Insert, Names, Create - make sure the left labels checkbox is ticked and click OK sample size 100 100 mean 50 50 std Dev 20 20 Error of mean 3.92 =1.96*std_Dev/SQRT(sample_size) Mean + Err 53.92 =mean+b5 Mean - Err 46.08 =mean-b5 new N 384.16 =(std_Dev*1.96/2)^2 so to reduce the error of the mean by half we increase the sample size by: =(std_Dev*1.96/2)^2 of course you have to get you mean and stdev first or make an estimate of them first. Hope this helps Peter "Ariel" wrote: Can someone give me the spreadsheet formulas for calculating a sample size for independent samples? Thanks Ariel |
calculating sample sizes for independent groups
Does that mean you can't help me, or that I need to clarify it further?
"Billy Liddel" wrote: Hi Ariel I think you lost me somewhere there! "Ariel" wrote: Hi Billy, Thank you for your effort, but this calculation is not for two independent groups. Assume that the mean and SD of the first group is 0.50 and 0.224, respectively. We estimate that the second group will have a mean value of 0.045 (difference between the two groups is 10%). I can even throw in that we estimate that the second group's SD will be 0.212. What will the sample size needed to acheive this in the second group, assuming a power of 80% and an alpha of 0.05.? An online calculator gave me a sample size of 29,864 if the two SDs are known (using the z-test) and a sample size of 31,506 when only the first SD is known and the second estimated. Thanks for your help! "Billy Liddel" wrote: I had to look this up, its ages since did this. It is based on the standard error of the mean. Say a sample of 100 items has a mean of 50 and a standard deviation of 20. The interval estimate with 95% confidence for the population mean is: =1.96*std_Dev/SQRT(sample_size) which gives four. The table below shows the figures and formulas. (I created Range names for the numbers to make it more clear. Select labels and figures and choose Insert, Names, Create - make sure the left labels checkbox is ticked and click OK sample size 100 100 mean 50 50 std Dev 20 20 Error of mean 3.92 =1.96*std_Dev/SQRT(sample_size) Mean + Err 53.92 =mean+b5 Mean - Err 46.08 =mean-b5 new N 384.16 =(std_Dev*1.96/2)^2 so to reduce the error of the mean by half we increase the sample size by: =(std_Dev*1.96/2)^2 of course you have to get you mean and stdev first or make an estimate of them first. Hope this helps Peter "Ariel" wrote: Can someone give me the spreadsheet formulas for calculating a sample size for independent samples? Thanks Ariel |
calculating sample sizes for independent groups
Hi
Sorry, I can't help you Peter "Ariel" wrote: Does that mean you can't help me, or that I need to clarify it further? "Billy Liddel" wrote: Hi Ariel I think you lost me somewhere there! "Ariel" wrote: Hi Billy, Thank you for your effort, but this calculation is not for two independent groups. Assume that the mean and SD of the first group is 0.50 and 0.224, respectively. We estimate that the second group will have a mean value of 0.045 (difference between the two groups is 10%). I can even throw in that we estimate that the second group's SD will be 0.212. What will the sample size needed to acheive this in the second group, assuming a power of 80% and an alpha of 0.05.? An online calculator gave me a sample size of 29,864 if the two SDs are known (using the z-test) and a sample size of 31,506 when only the first SD is known and the second estimated. Thanks for your help! "Billy Liddel" wrote: I had to look this up, its ages since did this. It is based on the standard error of the mean. Say a sample of 100 items has a mean of 50 and a standard deviation of 20. The interval estimate with 95% confidence for the population mean is: =1.96*std_Dev/SQRT(sample_size) which gives four. The table below shows the figures and formulas. (I created Range names for the numbers to make it more clear. Select labels and figures and choose Insert, Names, Create - make sure the left labels checkbox is ticked and click OK sample size 100 100 mean 50 50 std Dev 20 20 Error of mean 3.92 =1.96*std_Dev/SQRT(sample_size) Mean + Err 53.92 =mean+b5 Mean - Err 46.08 =mean-b5 new N 384.16 =(std_Dev*1.96/2)^2 so to reduce the error of the mean by half we increase the sample size by: =(std_Dev*1.96/2)^2 of course you have to get you mean and stdev first or make an estimate of them first. Hope this helps Peter "Ariel" wrote: Can someone give me the spreadsheet formulas for calculating a sample size for independent samples? Thanks Ariel |
calculating sample sizes for independent groups
Fri, 29 Jun 2007 08:38:01 -0700 from Ariel
: Assume that the mean and SD of the first group is 0.50 and 0.224, respectively. We estimate that the second group will have a mean value of 0.045 (difference between the two groups is 10%). I can even throw in that we estimate that the second group's SD will be 0.212. What will the sample size needed to acheive this in the second group, assuming a power of 80% and an alpha of 0.05.? Ask in a statistics group. The formula is pretty trivial, once you know what the calculation should be. -- Stan Brown, Oak Road Systems, Tompkins County, New York, USA http://OakRoadSystems.com/ |
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