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Peer Review

Interactive Distributions

by Ivo Dinov


Overall Numeric Rating:

4 stars
Content Quality: 4 stars
Effectiveness: 4 stars
Ease of Use: 4.8 stars
Reviewed: Mar 23, 2006 by Statistics Editorial Board
Overview: The interactive distributions section of the Statistics Online Computational
Resource (SOCR) provides an extensive series of interactive probability
distribution calculators. This Java based applet includes most of the standard
distributions such as binomial, beta, gamma and normal, but also some more
esoteric such as Gilbrats, Triangle or Rayleigh. These calculators are well
featured in that they produce a graphical representation of the distribution
along with calculation of the mean and variance of the distribution. It also
allows the user to select a point on the graphic and produce probabilities above
and below the selected point. Moving the mouse along the plot shows
probability/density values. Dragging edges of the shaded region shows
probability to right/left and between endpoints. The applet includes a snapshot
button that allows the user to take a screen shot of the applet.
Type of Material: Java applet.
Recommended Uses: This material can be used in a variety of in class and out of class senerios.
Some recommended uses include
(a) As a replacement for paper statistics tables to look up probabilities and
percentiles, and
(b) To allow students to visualize shapes of distributions as parameters change.
Technical Requirements: Java capable browser.
Identify Major Learning Goals: This material will help the user better understand probability distributions by
replacing textbook based "look-up" on stat tables with an applet that works for
any reasonable endpoint and parameter values. It will also assist in
visualizing ideas that stem from probability distributions such as p-values.
Target Student Population: Any level statistics student who would be using paper tables and familiar with
probabilites such as areas under density curves could use the applet. Many of
the distributions provided will only be recognized by students from more
advanced probability/mathematical statistics courses.
Prerequisite Knowledge or Skills: General idea of probability as areas under density curves - or sums from
probability functions.

Evaluation and Observation

Content Quality

Rating: 4 stars
Strengths: The key strengths of this applet include the consistent interface for all
distributions. It also includes depictions for an extremely lengthy list of

Concerns: In some discrete distributions (e.g. binomial, Poisson) the left/right/between
probabilities change continuously as the endpoint is dragged within the "bar"
for a single discrete value - as if it were a continuous variable. Automatic
axis rescaling makes it difficult at times to see the effect of changing a
parameter with a slider. A number of distribution options are listed in the
menu but not yet implemented.

Potential Effectiveness as a Teaching Tool

Rating: 4 stars
Strengths: The consistent interface allows students to "look-up" p-values and critical
values with the same basic tool for all distributions. This applet is good way
to easily "see" lots of distributions and how the parameters affect their shape.
This may allow courses to include more distributions than the traditional
binomial, normal, and t-distribution. For more advanced students, the links to
find out more about each distribution (from Wolfram's Mathworld) are a nice
Concerns: Long list of (40+) potential distributions may be intimidating to a beginner
student who might just need normal and t-distributions. Listing of (endpoint,
density function value) pair as one scrolls along a density curve might confuse
a student to read the density as a probability (as it is for a discrete random

Ease of Use for Both Students and Faculty

Rating: 4.8 stars
Strengths: Users can easily change the shaded area of a distribution by dragging endpoints.
This works fairly intuitively and will help students visualize the connection
between probabilities and the density/probability function. Sliders allow easy
specification/modifications of parameters.
Concerns: Mouse resolution can make it difficult to get an endpoint precisely. For
example, in finding a p-vlaue for a t.s.=2.18, student might have to "settle"
for endpoint of 2.172 or 2.19. This resource could be greatly enhanced by
allowing students to put in a specific value rather than trying to find it with
the mouse. This is especially important for students who will replace
traditional tables with this applet. Although you can enter values for
parameters in a text box you must hit enter to have it recorded. Just typing it
in and tabbing to different field or clicking on a different box appears to
leave the parameter unchanged - even though the typed value is changed. Default
parameters on some distributions are not reasonable - eg. p=0.0 is shown by
default for a geometric, when p-0.05 is apparently plotted.

The applet includes a snapshot feature that should take a screen shot of the
applet. However, the reviewers could not get this feature to work.

Other Issues and Comments: The use of paper tables has been necessary in statistics but has not had a good
alternative. The use of electronic "tables" like these is favored by many in
the statistics community. However, the question of how to give pencil and paper
exams without tables has yet to be solved. Instructors that spend a lot of
time with these type of applets should be sure to plan how they will give exams
to their students related to finding probabilities.