Sample Size Calculations – Correlations

Select menu: Stats | Sample Size | Correlations

Use this to calculate the number of replicates (sample size) or the power required for assessing the correlation between two variables within a single group of subjects, or between two groups of subjects. The correlation in this case is the product moment correlation coefficient, as calculated by the CORRELATION function (and so the variables are assumed to have Normal distributions). The calculations can be done for a one-sample t-test (testing for evidence that the mean of the sample differs from a specific value) or a two-sample test (testing that means of the samples are different).

From the menu select
Stats | Sample Size | Correlations.
Fill in the fields as required then click Run.

After running the calculation you can save the results by clicking Save.

Calculation

Specifies the calculation to be carried out: Sample size, or Power.

Test

Specifies the test to be carried out: One-sample, or Two-sample.

Correlation in sample

For a one-sample test this specifies the correlation, and the assumption is to assess whether this is non-zero. For a two-sample test this specifies the correlation within the first sample.

Correlation in sample 2

For a two-sample test this specifies the correlation for the second sample.

Significance level (alpha)%

Specifies the significance level at which the response is to be tested.

Probability of detection (power)%

For a sample size calculation this specifies the required probability for detection of the response (that is, the power of the test).

Sample size

For a power calculation this specifies the sample size to be used to calculate the probability of detection (power).

Replication ratio (sample 2: sample 1)

It is generally assumed that the sizes of the samples in the two-sample test should be equal. However, you can specify a ratio to indicate the size of the second sample in relation to the size of the first sample.

Power plot

When selected, produces a plot of power versus sample size. When Sample size is selected the plot is generated using 11 replication values centred around the required number of replicates for the requested power. When Power is selected you can specify a range of sample size values for the plot in the spaces provided.

Type of test

Lets you choose either a One-sided or Two-sided test.

Action Icons

	Pin	Controls whether to keep the dialog open when you click Run. When the pin is down the dialog will remain open, otherwise when the pin is up the dialog will close.
	Restore	Restore names into edit fields and default settings.
	Clear	Clear all fields and list boxes.
	Help	Open the Help topic for this dialog.