This dialog provides for sample-size determination for
estimating a mean to within a specified margin of error, for
either a finite population of specified size, or an infinite
population (or sampling with replacement).

The confidence interval is of the form y-bar +/- ME, where
y-bar is the sample mean and ME is the margin of error:

    ME = t * s / sqrt(n)                (infinite pop)

    ME = t * s * sqrt{(1 - n/N) / n}    (finite pop)

where t is a critical value from the t distribution, s is
the sample SD, n is the sample size, and N is the population
size.

USING THE GRAPHICAL INTERFACE
The dialog is designed such that a sample size n is computed
whenever you change any of the other input values.  If you
change n, a new ME is computed (using sigma in place of s in
the above formulas).

Finite population
If "Finite population" is checked, calculations are based on
the population size N entered in the adjacent input field.
If the box is unchecked, the "N" field is hidden, and
calculations are based on an infinite population.

Confidence
Choose the desired confidence coefficient for the ME.  This
determines the value of t.

Sigma
Enter your best guess at the population SD (based on
historical or pilot data).  For a finite population, sigma^2
is defined as the sum of squared deviations from the
population mean, divided by N-1.

Margin of Error
Enter the target value of the margin of error here.  Note
that the actual value achieved after you collect the data
will be more or less than this amount, because it will be
based on s (estimated from the data) rather than sigma.

n
Enter the sample size here if you want to see what margin of
error is achieved with that size sample.  When you change n,
it is rounded to the nearest integer; otherwise, n is not
rounded


Note on sliders
Sliders may be varied by clicking or dragging along the
center of the scale.  If you drag along at the level of the
tick labels, you can change the endpoints of the scale.  The
small button at the upper right of each slider converts it
to and from a keyboard-input mode.  In that mode, you can
also change the scale endpoints and number of digits
displayed.

Graphics
A simple graphics facility is available from the Options
menu.  Enter the variables for the x and y axes, and the
range of values for the x variable.  More information is
available through the graphics dialog's help menu.
