Confidence interval for a 2-group mean difference

Source: R/statpsych1.R

Computes equal variance and unequal variance confidence intervals for a population 2-group mean difference using the estimated means, estimated standard deviations, and sample sizes. Also computes equal variance and unequal variance independent-samples t-tests. Use the t.test function for raw data input.

ci.mean2(alpha, m1, m2, sd1, sd2, n1, n2)

Arguments

alpha

alpha level for 1-alpha confidence

m1

estimated mean for group 1

m2

estimated mean for group 2

sd1

estimated standard deviation for group 1

sd2

estimated standard deviation for group 2

n1

sample size for group 1

n2

sample size for group 2

Value

Returns a 2-row matrix. The columns are:

  • Estimate - estimated mean difference

  • SE - standard error

  • t - t test statistic

  • df - degrees of freedom

  • p - two-sided p-value

  • LL - lower limit of the confidence interval

  • UL - upper limit of the confidence interval

References

Snedecor GW, Cochran WG (1989). Statistical Methods, 8th edition. ISU University Pres, Ames, Iowa.

Examples

ci.mean2(.05, 15.4, 10.3, 2.67, 2.15, 30, 20)
#>                              Estimate        SE        t       df            p
#> Equal Variances Assumed:          5.1 0.7151214 7.131656 48.00000 4.621279e-09
#> Equal Variances Not Assumed:      5.1 0.6846568 7.448987 46.17476 1.898214e-09
#>                                    LL       UL
#> Equal Variances Assumed:     3.662152 6.537848
#> Equal Variances Not Assumed: 3.721998 6.478002

# Should return:
#                              Estimate       SE        t      df      
# Equal Variances Assumed:          5.1 1.602248 3.183029 48.0000 
# Equal Variances Not Assumed:      5.1 1.406801 3.625247 44.1137 
#                                          p       LL       UL
# Equal Variances Assumed:      0.0025578586 1.878465 8.321535
# Equal Variances Not Assumed:  0.0007438065 2.264986 7.935014