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reason to believe that the two unknown population variances are different, we
can use the following confidence interval
2
2/,21
2
2/,21
2121
)(,)(
XXXX
stXXstXX
±±
⋅+±⋅−±
αναν
where the estimated standard deviation for the sum or difference is
2
2
2
1
2
1
21
n
s
n
s
s
XX
+=
±
and n, the degrees of freedom of the t variate, are calculated using the integer
value closest to
)]1/()/[()]1/()/[(
)]/()/[(
22
2
211
2
1
2
2
2
21
2
1
−+−
+
=
nnSnnS
nSnS
ν
Determining confidence intervals
The application 6. Conf Interval can be accessed by using ‚Ù—
@@@OK@@@. The application offers the following options:
These options are to be interpreted as follows:
1. Z-INT: 1 µ.: Single sample confidence interval for the population mean,
µ, with known population variance, or for large samples with unknown
population variance.
2. Z-INT: µ1−µ2.: Confidence interval for the difference of the population
means, µ
1
- µ
2
, with either known population variances, or for large
samples with unknown population variances.
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