Tukey                package:Devore5                R Documentation

_P_e_r_f_o_r_m _T_u_k_e_y _m_u_l_t_i_p_l_e _c_o_m_p_a_r_i_s_o_n_s

_D_e_s_c_r_i_p_t_i_o_n:

     Create a set of confidence intervals on the differences between
     the means of the levels of a factor with the specified family-wise
     probability of coverage.  The intervals are based on the
     Studentized range statistic.

_U_s_a_g_e:

     Tukey(x, which=seq(along = tabs), ordered=FALSE, conf.level=0.95, ...)

_D_e_t_a_i_l_s:

     When comparing the means for the levels of a factor in an analysis
     of variance, a simple comparison using t-tests will inflate the
     probability of declaring a significant difference when it is not
     in fact present.  This because the intervals are calculated with a
     given coverage probability for each interval but the
     interpretation of the coverage is usually with respect to the
     entire family of intervals.

     John Tukey introduced intervals based on the range of the sample
     means rather than the individual differences.  The intervals
     returned by this function are based on this Studentized range
     statistics.

     Technically the intervals constructed in this way would only apply
     to balanced designs where there are the same number of
     observations made at each level of the factor.  This function
     incorporates an adjustment for sample size that produces sensible
     intervals for mildly unbalanced designs.

_V_a_l_u_e:

     A list with one component for each term requested in `which'. Each
     component is a matrix with columns `diff' giving the difference in
     the observed means, `lwr' giving the lower end point of the
     interval, and `upr' giving the upper end point.

_A_u_t_h_o_r(_s):

     Douglas Bates bates@stat.wisc.edu

_S_e_e _A_l_s_o:

     `qtukey', `model.tables'

_E_x_a_m_p_l_e_s:

     data(xmp10.01)
     fm1 <- aov(strength ~ type, data = xmp10.01)
     Tukey(fm1)
     Tukey(fm1, ord = TRUE, conf = 0.9)

