Summary statistics for circular data
circsummarize varlist [weight] [if exp] [in range] [, ci level(#) rayleigh kuiper detail ]
circsummarize varname [weight] [if exp] [in range] [, by(byvar) ci level(#) rayleigh kuiper detail ]
fweights and aweights are allowed; see help weights. If weights are specified, they are used in calculating the mean direction and the vector strength (and hence the Rayleigh test and confidence limits if produced, although this may not be justifiable). Weights are not used in calculating the circular range or the Kuiper test.
The abbreviation circsu (only) is allowed.
circsummarize produces summary statistics for circular variables with scales between 0 and 360 degrees. These are by default
the mean direction (in degrees),
the vector strength or mean resultant length (i.e. length of resultant / number of observations)
and the circular range (length of smallest arc including all observations) (in degrees).
Suppose we have n measurements of a circular variable t. Then calculate in turn C = SUM cos t S = SUM sin t arctan(S / C), the mean direction R = sqrt(C * C + S * S), the length of the resultant vector R / n, the vector strength or mean resultant length.
by() specifies that results are to be shown for each group defined by values of byvar. This option is only available if a single varname is specified.
ci produces a confidence interval for the vector mean suitable for large samples (at least 25 values, say). This is based on an assumption that sampling variation follows a normal distribution. If we calculate further
m_2 = (1 / n) SUM cos 2(t - vector mean of t),
then the circular standard error CSE is estimated by
sqrt[ n * (1 - m_2) / (2 * R * R)
and the confidence interval is estimated as
(vector mean - arcsin(z * CSE), vector mean + arcsin(z * CSE)),
where z is the appropriate percentage point, given level(), from the Normal distribution with mean 0 and standard deviation 1.
level() specifies the confidence level in percent for the confidence interval. See help for ci.
rayleigh produces the Rayleigh test, which tests a null hypothesis of uniformity against an alternative hypothesis of unimodality. The resulting P-value is shown.
kuiper produces the Kuiper test, which tests a null hypothesis of uniformity against any alternative. The resulting P-value is shown. A numerical approximation due to Stephens (1970, p.118) is used that gives accuracy to 3 decimal places for P < 0.447.
detail is a synonym for ci rayleigh kuiper.
. circsummarize axisasp wallasp
. circsummarize wallasp, by(grade) detail
Fisher, N.I. 1993. Statistical analysis of circular data. Cambridge: Cambridge University Press.
Stephens, M.A. 1970. Use of the Kolmogorov-Smirnov, Cramér-von Mises and related statistics without extensive tables. Journal, Royal Statistical Society Series B 32: 115-22.
Nicholas J. Cox, University of Durham, U.K. email@example.com
On-line: help for ci