The StatCrunch Correlation by eye applet has been designed to help students learn a better understanding of how various data values affect correlation. This interactive demonstration generates scatter plots at various correlations. This applet allows for the guessing of the sample correlation for various datasets and will give messages based on how far the guess is from the sample correlation.
Click on StatCrunch > Applets > Correlation by eye to view the dialog shown above. One may choose to either populate the applet using randomly generated data or to use data from the StatCrunch data table. If using data table is selected, one can specify a Where statement can be specified to control what observations will be included. As in most applets, there is an option for a custom Title to override the default applet title.
Clicking on an unoccupied region of the graph will add a point at the location clicked. Existing points can be clicked and dragged to a new location on the graph. To delete a point, drag it into the trash bin on the far right. Any change on the graph results in a change of the sample correlation.
The top button menu offers various other options for changing the applet. The Sample size input allows for the selection of one of the default sample sizes or for entering any integer size up to 500. The change in sample size is reflected the next time data is generated using the Simulate button. The Add point feature offers the ability for adding a specific data point outside of the visible range in which case the range of the graph will be updated. This is particularly useful when showing the effect of an extreme point on the sample correlation.
In the table at the bottom of the applet is a blank white box that will accept a Guess for the sample correlation. The message above the guess box will change depending on how close the guess is to the sample correlation. The second row of the table is another blank box that will be filled by toggling the Show button. This displays the sample correlation for the displayed scatterplot. As long as the Show button is toggled down, the sample correlation will remain in the bottom middle box of the table.
One usage of this applet is teaching the concept of correlation through the interactive guessing. Guessing at the sample correlation for various datasets teaches what correlation means in practice. If teaching a class, this can become a game with students by asking for a guess and then seeing how close they get to the sample correlation. Another potential use is through showing how extreme points have a weighted effect on the sample correlation at various sample sizes. Create a clear linear trend, then place a point completely off that trend to notice how dramatic the shift in correlation can be due to one point.
Note that the applet can also be saved to you’re My Results folder at statcrunch.com. When the applet is saved using the Options > Export to My Results option of the applet window, the applet will then be saved in its current form with the existing data values. If one so chooses, the result can then be shared with others by clicking the Edit link on the resulting statcrunch.com result page. One can then share the link to the result page with other users. This may be an enticing option for those who want to construct prepackaged applets for classroom usage.