In an investigation of fruit-fly behavior, a covered choice chamber is used to test whether the spatial distribution of flies is affected by the presence of a substance placed at one end of the chamber. To test the flies' preference for glucose, 60 flies are introduced into the middle of the choice chamber at the insertion point indicated by the arrow in the figure above. A cotton ball soaked with a 10 percent glucose solution is placed at one end of the chamber, and a dry cotton ball with no solution is placed at the other end. The positions of flies are observed and recorded every minute for 10 minutes.
(a) Prediction and Justification 2 points maximum
Predict the distribution of flies in the chamber after 10 minutes and justify your prediction.
- 1 point for predicting the location of the flies in the choice chamber
- 1 point for justifying the prediction
(b) Improving the Experimental Design 4 points maximum
Propose ONE specific improvement to each of the following parts of the experimental design and explain how the modification will affect the experiment.
(c) Banana Preference — Chi-Square Test 2 points maximum
The experiment described above is repeated with ripe bananas at one end and unripe bananas at the other end. Once again, the positions of the flies are observed and recorded every minute for 10 minutes. The positions of flies after 1 minute and after 10 minutes are shown in the table below.
| Time (minutes) | End with Ripe Banana | Middle | End with Unripe Banana |
|---|---|---|---|
| 1 | 21 | 18 | 21 |
| 10 | 43 | 3 | 12 |
Perform a chi-square test on the data for the 10-minute time point in the banana experiment. Specify the null hypothesis that you are testing and enter the values from the table above in the input fields below to perform your calculations.
Working Table
| Category | Observed (O) | Expected (E) | O − E | (O − E)² | (O − E)² / E |
|---|---|---|---|---|---|
| Ripe | — | — | — | — | — |
| Middle | — | — | — | — | — |
| Unripe | — | — | — | — | — |
| χ² = | — | ||||
Null Hypothesis and Alternative
H0: Fruit flies are distributed equally among the three regions (ripe end, middle, unripe end) after 10 minutes; any differences are due to chance.
HA: Distribution differs from equal proportions (preference or avoidance is present).
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