unless you had a table comparable to the z table for area under the normal distribution ... for EACH different level of skewness ... an exact answer is not possible in a way that would be explainable
here is one example that may help to give some approximate idea as to what might happen though . .:::. .:::::. .:::::::. .:::::::::. .:::::::::::. ::::::::::::::: . .....:::::::::::::::::::..... . +---------+---------+---------+---------+---------+-------C1 the above is a norm. distribution of z scores ... where 1/2 the data are above 0 and 1/2 are below : :. .::. ::::. :::::. ::::::::. ::::::::::::................. ... . +---------+---------+---------+---------+---------+-------C2 -5.0 -2.5 0.0 2.5 5.0 7.5 here is a set of z score data that is radically + skewed ... and even though it has 0 as its mean, 50% of the area is NOT above the mean of 0 ... note the median is down at about -.3 ... so, there is LESS than 50% above the mean of 0 ... this means that for z scores above 0 ... there is not as much area beyond (to the right) ... as you would expect if the distribution had been normal ... so, we can have some approximate idea of what might happen but the exact amount of this clearly depends on how much skew you have MTB > desc c1 c2 Descriptive Statistics: C1, C2 Variable N Mean Median TrMean StDev SE Mean C1 10000 0.0008 0.0185 0.0029 1.0008 0.0100 C2 10000 0.0000 -0.3098 -0.1117 1.0000 0.0100 Variable Minimum Maximum Q1 Q3 C1 -3.9468 3.9996 -0.6811 0.6754 C2 -0.9946 8.0984 -0.7127 0.3921 At 07:27 AM 1/30/02 -0800, Melady Preece wrote: >A student wants to know how one can calculate the area under the curve for >skewed distributions. Can someone give me an answer about when a >distribution is too skewed to use the z table? > >Melady _________________________________________________________ dennis roberts, educational psychology, penn state university 208 cedar, AC 8148632401, mailto:[EMAIL PROTECTED] http://roberts.ed.psu.edu/users/droberts/drober~1.htm ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =================================================================