You can arrive at this number by sheer reasoning but looking at the above probability distribution makes the task very easy. We can relate a range of outcomes to their probabilities. In general, a probability distribution helps one test hypotheses about a population. The 90 th percentile is the number that divides the distribution such that 90 percent of the cases are above it and 10 percent below. Two commonly used percentiles are the upper and lower quartile.
Most statistical programs report quartiles along with the median and mean.
The difference between the upper and lower quartiles is called the interquartile range. Not of any great use now. Percent of all persons in the county who are poor. Percent of children 5 to 17 who are poor Median household income. Here are some statistics for percent of persons in the county who are poor.