Loggerhead Turtles Question #2

Scoring 4Complete 3Substantial 2Developing 1Minimal 0

Intent of Question The primary goals of this question were to assess a student's ability to (1)describe a Type I error and its consequence in a particular study; (2) draw an appropriate conclusion from a p-value; and (3) describe a flaw in a study and its effect on the conclusions that can be drawn from the study.

Question #2

Part a a) Describe what a Type I error would be in the context of the study, and also describe a consequence of making this type of error. Part a solution In the context of the study, a Type I error means concluding that the mean number of eggs that successfully hatch per nest is higher for nests with predator cages than for nests without predator cages when, in reality, there is no difference in the fledgling rates for nests with and without cages. The consequence of this error is that the National Park Service will receive funding for the predator cages when they are not actually keeping predators away from the turtle nests, thus wasting money that could be more effectively spent.

Scoring Parts (a), (b), and (c) are scored as essentially correct (E), partially correct (P), or incorrect (I). Part (a) is scored as follows: Essentially correct (E) if the response correctly includes the following two components: 1) Describes the error in context by referring to the difference in mean number of eggs hatched for nests with and without cages. 2) Describes a consequence of the study as spending funds on a method that is not effective and/or the continued stealing of eggs by predators. Partially correct (P) if the response correctly includes only one of the two components listed above. Incorrect (I) if the response correctly includes neither of the two components listed above.

Part a scoring notes If a response provides more than one description of a Type I error, score the weakest attempt. Referring to the symbolic hypotheses is not sufficient for context. If a response describes a Type I error incorrectly, the response can get the consequence component correct if it is consistent with the incorrectly described error. If a response gives an incomplete description of a Type I error (for example, "we reject the null hypothesis of no difference between the mean number of eggs hatched for nests with and without cages"), the response can get the consequence component correct if the consequence is consistent with the partial description of the error. If a response provides no description of a Type I error, the response cannot get the consequence component correct. Describing the Type I error only in terms of the consequence (for example, "They approve funding for the cages when they should not") should get credit for the consequence but should not get credit for the error, because there is no reference to the mean number of eggs hatched for nests with and without cages.

Question #2 - continued Each season, the Park Service moves approximately half of the turtle nests at Cape Lookout very soon after they are laid because they are laid in locations that are vulnerable to extreme high tides. The Park Service decides to collect data for their study by randomly selecting 35 of the nests that were moved and placing a cage over them and comparing the fledgling rate to the rate for 35 randomly selected nests that were neither moved nor caged. This resulted in a p-value of for the hypotheses stated above. If it was reasonable to conduct a test of significance for the hypotheses stated above using the data collected, what would the p-value of lead you to conclude?

Part b solution Because the p-value of is less than  = 0.05, we reject the null hypothesis. There is convincing statistical evidence the mean number of eggs that successfully hatch per nest is greater for nests with a predator cage than for nests without a predator cage for the population of loggerhead turtle nests on Cape Lookout.

Scoring Parts (a), (b), and (c) are scored as essentially correct (E), partially correct (P), or incorrect (I).

Part b scoring notes

Question #2 - continued Part c Describe the primary flaw in the study described in part (b), and explain why it is a concern. Part c - solution This study did not incorporate randomization into the assignment of experimental units to treatments, in this case in the assignment of nests to being caged or not caged. Because only nests that were moved received cages, it becomes impossible to distinguish whether the observed differences in fledgling rates were due to the treatment (the predator cages) or due to factors related to the movement of the nests.

Scoring Parts (a), (b), and (c) are scored as essentially correct (E), partially correct (P), or incorrect (I) Part (c) is scored as follows: Essentially correct (E) if the response correctly includes the following two components: 1) States that the experiment did not incorporate random assignment. 2) Describes how confounding is present in this experiment due to the lack of randomization. Partially correct (P) if the response correctly includes only one of the two components listed above. Incorrect (I) if the response correctly includes none of the components listed above.

Part c scoring notes Notes: If for the first component a response provides additional proposed flaws (for example, "the sample size is too small"), score the weakest attempt. Simply referring to "confounding" is not sufficient for the second component unless the concept of confounding is clearly explained. Saying "the results cannot be generalized" or "the results will be inaccurate" does not describe how confounding is present and is not sufficient for the second component. Incorrect application of statistical concepts (for example, saying that the results will be "skewed") results in a loss of credit for the second component.

Scoring 4Complete Response All three parts essentially correct 3Substantial Response Two parts essentially correct and one part partially correct 2Developing Response Two parts essentially correct and one part incorrect OR One part essentially correct and one or two parts partially correct OR Three parts partially correct 1Minimal Response One part essentially correct and two parts incorrect OR Two parts partially correct and one part incorrect