ppoints                package:stats                R Documentation

_O_r_d_i_n_a_t_e_s _f_o_r _P_r_o_b_a_b_i_l_i_t_y _P_l_o_t_t_i_n_g

_D_e_s_c_r_i_p_t_i_o_n:

     Generates the sequence of "probability" points '(1:m - a)/(m +
     (1-a)-a)' where 'm' is either 'n', if 'length(n)==1', or
     'length(n)'.

_U_s_a_g_e:

     ppoints(n, a = ifelse(n <= 10, 3/8, 1/2))

_A_r_g_u_m_e_n_t_s:

       n: either the number of points generated or a vector of
          observations.

       a: the offset fraction to be used; typically in (0,1).

_D_e_t_a_i_l_s:

     If 0 < a < 1, the resulting values are within (0,1) (excluding
     boundaries). In any case, the resulting sequence is symmetric in
     [0,1], i.e., 'p + rev(p) == 1'.

     'ppoints()' is used in 'qqplot' and 'qqnorm' to generate the set
     of probabilities at which to evaluate the inverse distribution.

     The choice of 'a' follows the documentation of the function of the
     same name in Becker _et al_ (1988), and appears to have been
     motivated by results from Blom (1958) on approximations to expect
     normal order statistics (see also 'quantile').

_R_e_f_e_r_e_n_c_e_s:

     Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) _The New S
     Language_. Wadsworth & Brooks/Cole.

     Blom, G. (1958) _Statistical Estimates and Transformed Beta
     Variables._ Wiley

_S_e_e _A_l_s_o:

     'qqplot', 'qqnorm'.

_E_x_a_m_p_l_e_s:

     ppoints(4) # the same as  ppoints(1:4)
     ppoints(10)
     ppoints(10, a=1/2)

