An experiment examined the impact of THC (the active ingredient in marijuana) on various physiological and psychological variables. The study recruited a sample of 18 young adults who were habitual marijuana smokers. Subjects came to the lab 3 times, each time smoking a different marijuana cigarette: one with 3.9% THC, one with 1.8% THC, and one with no THC (a placebo). The order of the conditions was randomized in a double-blind design.
At the start of each session, no subject reported being "high." After smoking the cigarette, participants rated how "high" they felt, using a positive continuous scale (0 representing not at all "high"). For the placebo condition, participants reported a mean "high" feeling of 11.3, with a standard deviation of 15.5. Is there evidence of a significant placebo effect, with subject feeling significantly "high" after smoking a placebo marijuana cigarette?
Which of the following statements is the correct interpretation for a 95% confidence interval?

Question

Quizplus · Accepted Answer

The correct interpretation of a 95% confidence interval is that we are 95% confident that the population mean level of "high" is between 3.79 and 18.81. This means that if we were to repeat this experiment many times and calculate a 95% confidence interval each time, 95% of those intervals would contain the true population mean. Choice A is incorrect because it implies that we are making a statement about individual observations, rather than the population parameter. Choice B is incorrect for the same reason, as well as the fact that it suggests we are making a probabilistic statement about a single observation, which is not appropriate for a confidence interval. Choice C is incorrect because it suggests we are only making a statement about the sample mean, which is not the same as the population mean.

An Experiment Examined the Impact of THC (The Active Ingredient