Help finding the variables in this experiment?

Question

For example: A team of researchers observed that the acorn density of the oak forests in New York fluctuates from year to year and that mice populations in the forest were increased when the acorn density was high (Jones, Ostfeld and Wolff, 1996). These researchers asked, "If there are more acorns and mice in the forest, will there be a higher density of ticks that carry Lyme Disease?" They hypothesized that the number of ticks carrying Lyme disease would be high when acorn density in the forest was high.
To test their hypothesis, the researchers studied six unfenced oak forest plots in New York. Mice were initially removed from each plot by trapping. Four tons of acorns were added to three of the plots (with the help of some local girl scouts!). The remaining three plots did not receive acorns. Ticks were collected in 225 sections (15m X 30m) in each plot, and were identified and counted. The number of mice in each plot was counted also.

I get that the dependent variable is: the number of mice and ticks?

And the independent variable is: the number of acorns

But, I am really lost on determining the level of treatment and number of replicates? Any help is appreciated. Would the number of replicates be three or six in this case?

How can you tell what the sample size is in an experiment like this?

 

Answer 1

Determining the variables, number of replicates, and sample size in an experiment:

Variables:

You are partially correct with regard to the independent and dependent variables in this experiment. The independent variable is the variable that the researchers are directly controlling. In this case, the researchers are directly controlling whether or not a given plot has acorns, thereby testing their hypothesis that acorn density impacts the number of ticks carrying Lyme disease. Based on this, I would say that the independent variable is the presence or absence of acorns, which could be considered acorn density to fit the hypothesis they are trying to test.

Dependent variables are the variables that are being measured in response to the independent variable. These variables provide the results of the study, and they cannot be directly controlled by the researchers. This experiment was testing the impact of acorn density on the number of ticks present in each plot, so the dependent variable is the number of ticks.

Confounding variables are variables that may impact the results of the study but are not being controlled by the researchers. In this case, the number of mice in each plot is a confounding variable since the researchers are not controlling the number of mice, but the number of mice may impact the number of ticks. The number of mice is not a dependent variable because the researchers are asking how acorn density impacts the number of mice, and their hypothesis does not say anything about mice.

Replicates:

The replicates in an experiment like this refers to how many times the experiment was repeated for each condition. In this case, 225 sections of land were sampled in a given plot, and this was repeated 3 times for each treatment condition. This means there are 3 replicates for each group.

Sample size:

Sample size in biology/ecology refers to how many samples are included in each experimental group. In this experiment, 225 sections of land were sampled in each plot to determine the number of ticks. This means that there are 225 x 3 samples per condition (675 samples).