How do I use statistics?
The main screens Stats menu contains the Sample Size and Margin of Error options. Other statistics are available in analysis.
Sample Size lets you determine the right sample size for your work and Hypothesis Testing lets you test whether two groups are statistically the same or whether chance can explain the observed differences.
Sample Size / Margin of Error:
If you want to know the minimum sample size required to supply a specified level of confidence and accuracy, use the Sample Size option. For the hypothesis you are testing you will need to estimate the proportion of the population giving a response. For example, if you expect somewhere between 20 to 30 percent of the population to give a specific response to your question, then Sample Size will show you the minimum sample size if 20 percent respond, or 30 percent. The larger of the two sample sizes will be the one that would assure you of the desired precision.
Supply first the level of confidence and the margin of error that you want your sample size to attain. Then for each value of response in favor that you supply, Sample Size will show you the minimum sample required to achieve that level of accuracy.
Basically, you will supply three pieces of information (responses in favor, margin of error, and level of confidence). The Sample Size command then provides the minimum sample size required to meet those three conditions.
Sample Size is arranged in convenient fashion to let you change any one of your input parameters, with the result displayed on the same screen. You are able to quickly change parameters to study different scenarios. This command very efficiently lets you explore the implications of different parameters. Below is the Sample Size screen.
You may display multiple scenarios at the same time by selecting Edit | Add column from the menu. You may have as many scenarios as fit on the screen.
Sample size / Margin of Error Definitions:
A hypothesis test compares the means of two groups to determine whether they come from the same population, at some given level of confidence.
The summary results are displayed in the screen template. Initially, they reflect the results from the preset values of the parameters. You may change the Confidence Level, Hypothesis Value, Sample Size, and Response Rate parameters as appropriate to your test.
Hypothesis Value is the difference between the two population means (Group A minus Group B). Its preset value is zero, implying that the two populations are the same. As an example, if the hypothesis were that Group A is larger than Group B by a value of 10, then the number 10 would be supplied for this value.
The Confidence Level is default set at a value of 90 percent. You may want to change this value to 95 or 99 percent if you want a higher level of confidence.
The Type of Test is set to two-sided test. If you want to change this to either a left-sided or right-sided test, depending upon the hypothesis, then select the appropriate levels of confidence to match your hypothesis. (Remember that the hypothesized value is testing for Group A minus Group B when you select for left-sided or right-sided test.)
For number values, you will need to supply additional values for each of the two groups, Group A and Group B. These will be Sample Mean and Standard Deviation/Standard Error.