Step 1: Find ?/2
Level of Confidence = 90%
? = 100% - (Level of Confidence) = 10%
?/2 = 5% = 0.05
Step 2: Find degrees of freedom and t?/2
Degrees of freedom = smaller of (n1 - 1 , n2
- 1 ) = smaller of (16 , 7) = 7
Calculate t?/2 by using t-distribution with degrees of
freedom (DF) = 7 and ?/2 = 0.05 as right-tailed area and
left-tailed area.
Step 3: Calculate Confidence Interval
t?/2 = 1.89455
Standard Error = ? (s?)²/n? + (s?)²/n? = ?1.5153 = 1.2309
Lower Bound = (x?? - x??) - t?/2•(? (s?)²/n? + (s?)²/n?
) = (122 - 97) - (1.89455)(1.2309) = 22.6678
Upper Bound = (x?? + x??) + t?/2•(? (s?)²/n? + (s?)²/n?
) = (122 - 97) + (1.89455)(1.2309) = 27.3321
Confidence Interval = (22.6678174147, 27.3321825853)
Interpretation of a confidence interval:
Since we do not know if the confidence interval (22.6678, 27.3321)
contains (?? - ??) or not, we are only 90% confident that (22.6678,
27.3321) contains (?? - ??).