Good News! These calculators have earned their own website! We'll leave these calculators here for now, but for our latest calculators please visit
Each calculator has four panels, similar to those shown here on the right.
The contents of these panels are hidden by default, but you can click to view: notation, descriptive information, literature reference ... and for the stat-geek-in-you, the underlying mathematical equation, and R code.
When you view the calculator, the form is pre-filled with default values; an accompanying graph is also displayed.
You may fill in the form with whatever values you like ... with obvious restrictions such as numbers must not contain text, variances must be positive, probabilities must be between 0 and 1, etc. All inputs are required, except of course sample size, which is at the top of the form, and displays the calculated sample size required for the given input values. After filling in the input values, click the button at the bottom of the form to see the new sample size and graph. The calculators always round up the calculated sample size to the nearest whole number, which may create a "step" pattern in the graph.
The graph is intended to provide you with a nice visual aid of how sensitive the required sample size is to the input parameter values. The y-axis (vertical) of the graph is always sample size, and the x-axis (horizontal) can be chosen with the drop-down menu below the graph. You may adjust the range of the x-axis in two ways: use the slider under the graph; enter values in the "min" and "max" boxes and then hit "resize".
Currently, the graph will always display 2 or 3 curves, each one representing the sample-size curve for different choices of power. The legend above the graph identifies which curve goes with which power level. As you mouse-over the graph the legend will show the calculated sample size and x-value as you move the mouse over the graph.