lecture 8 distributions percentiles and boxplots practical psychology 1
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Lecture 8Distributions
Percentiles and Boxplots
Practical Psychology 1
The Boxplot
Once these values have been calculated, this
information can be used to draw a boxplot: Also known as a “box-and-whisker” plot
Quick and easy method of checking distribution
normality/ skewness.
You need to know how to: draw boxplots by hand
Produce them in SPSS
and interpret them.
Boxplot and shape of distribution:
normal distribution
If data is normallydistributed, the boxplot
issymmetrical (i.e. the MEDIAN line isvery close to the centreof the rectangle):
Boxplot and shape of distribution:
positive skew
Note MEDIAN Position: there is a greater proportion of data on the lower end of the scale
Note MEDIAN Position:there is a greater proportion of data on the upper end of the scale
Boxplot and shape of distribution: negative skew
Boxplots in SPSS
Example: comparing male and female scores
Producing a Boxplot in SPSS
SPSS menu Graphs Legacy Dialogs Boxplot Simple Summaries of groups of cases Define
Drag the continuous
variable to the
“Variable box” and
the categorical
(e.g. gender) to
the “Category
Axis” box.
Each group is represented by a rectangle, in which 50% of the scores lie (this is the interquartile range, IQR)
Y axis = scores (DV)
The central line is the MEDIAN (Q2)
X-axis: (IV) group: males vs. females
Some terminology
H-spread = IQR (i.e., Q3-Q1)
Upper Whisker = largest value
Median (Q2)
Lower Whisker = smallest value
Lower Quartile (Q1) {lower Hinge}
Upper Quartile (Q3) {upper Hinge}
Percentiles in SPSS
Analyze >Descriptive Statistics > Explore
Put the IV (gender) in the factor list and the DV (scores) in the dependent list
Menu on the right: Statistics > tick “Percentiles”
“Continue”
Percentiles Output