This applet displays various distributions and allows the user to experiment with the parameters to see the effects on the curve.
Content Quality 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.
Content Quality Strengths:
The key strengths of this applet include the consistent interface for all distributions. It also includes depictions for an extremely lengthy list of distributions.
Ease of Use 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.
Ease of Use 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.
Potential Effectiveness 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 variable).
Potential Effectiveness 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 touch.
Potential Effectiveness Rating:
Source Code Available:
Source Code Available