Some
Formulas for Methods of Research
If you are given a sample size and need to know the margin of
error
use M = 1/sqrt(N). M in this case is the Margin
of Error as a
proportion (not a percent). For your final answer, convert the
result
to a percent.
For a sample with several sub-groups, you do the
computation
separately with sor each sub group with "N" being the number of people
in each sub group. Then you add the totals together.
If you need a margin of error for a mean score (an average such as
income in dollars or scores on a test), you need to know the standard
deviation (sd) and the sample size (N). Ignore any other information
you are given, including the size of the population.
Use the following formula: M = 2 * sd / SQRT(N)
If you are given a margin of error and asked how large a sample
you
need, use N = 1/ M(squared). If you have subgroups, you
need a sample
of size N for each sub group, the total sample size being all the
subsamples added up. M is the margin of error expressed as
a proportion, not a percent.
To compute a standard deviation, do the following:
1. compute the mean for the distribution (add up all
the scores and divide the total by N, the number of cases).
2. compute the difference between each score and the mean
3. square these differences
4. add up all the squared differences
5. divide the sum of the squared differences by the number
of cases, minus one
6. take the square root of the result from line 5
The following formula says the same thing.
