1. Is there a graphical method for finding the quartiles in a set of data?
The quartiles and median can easily be found from a cumulative frequency graph. See topic 53, Statistical Graphs.
2. How are the units decided for the stem of a stem and leaf diagram.
It depends on the data. If all of the numbers are between 250 and 320 then the stem could be "tens" i.e. from 25
3. Can the median, quartiles and percentiles be found on a stem and leaf graph?
Yes. You have to work out the position of the median in the list of values.
e.g. If there are 21 values, the median is the 11th value. You then count in to the 11th value from the highest end or the lowest end of the diagram.
Use a similar method for the quartiles and percentiles.