When random samples are drawn from a normal population, the sample skewness coefficient will fall within the ranges shown in the table below 90% of the time. If your sample skewness coefficient lies outside of the specified range, it is appropriate to conlcude that the population is significantly skewed, using the decision rule shown below.
 

Decision Rule

        <----------                                                                    ------------>
         Population                 Population may be                        Population
         skewed left                    symmetric                                skewed right
         ---------------------------------------------------------------------
         Lower                                0                                            Upper
         Limit                                                                                 Limit
 

Skewness Table

Sample size          Lower Limit        Upper Limit
25                           -.711                   +.711
30                           -.622                   +.622
35                           -.621                   +.621
40                           -.587                   +.587
50                           -.534                   +.534
75                           -.446                   +.446
100                         -.389                   +.389
150                         -.321                   +.321
200                         -.280                   +.280
300                         -.230                   +.230
400                         -.200                   +.200
500                         -.179                   +.179