PSYC 204- Lecture #5

PSYC 204- Lecture #5

1. Interquartile Range

   • IQR indicates the spread of the middle half of the data. The 50% range of your data, distance from 3rd quartile to 1st quartile, shows how data is scattered
   • IQR = Q3-Q1
   • method
     • A score can be the outlier if it is less than (1.5)(IQR) below the first quartile or (1.5)(IQR) above the third quartile.

2. Percentiles

3. Box Plots (Box -and-Whisker Plots)

   • Need five values to construct: the minimum value, Q1, median, Q3 and the maximum value
   • red line = median
   • minimum = beginning of box
   • maximum = end of box

4. calculate median

   • To calculate median, you should use cum f.
   • Q1: What if we have continuous variable with class interval different than 1?
     • median = M = L + ( (N/2) -Fb) / fm * h
       • L= exact lower limit of interval containing the median
       • Fb = cumulative frequency up to, but not including, the median interval
       • Fm = f. in the median interval
       • h = interval width (upper exact limit — lower exact limit)
       • N = number of cases or sample size