The students concluded that pea plants grew better in compost than did melon plants. Which of the following best addresses the validity of the conclusion made by the students? a) the conclusion is valid because the independent and dependent variables in the experiment were controlledb) the conclusion is valid because the experimental design included a large sample sizec) the conclusion is invalid because other variables in the experiment affected the resultsd) invalid because the units of measurement were not given in the experimental design

c. The conclusion is invalid because other variables in the experiment affected the results.

Explanation:

Consider the complete problem:

A group of students designed an experiment to determine the effect of compost on the germination and growth of plants.

The students set up experimental plots by mixing different ratios of soil and compost. They planted 20 pea and 20 melon

seeds in each plot and watered each plot regularly. The students recorded the number of seeds that germinated, and as the

plants grew they recorded the plant length, number and size of leaves, and general health observations.

The following observations were recorded.

 In all treatments, more pea seeds germinated than melon seeds

 There was a week of rainy weather at 3 weeks

 Melon plants developed a fungal growth at four weeks

 Melon plants grew longer than pea plants, but many melon leaves showed signs of yellowing as the weeks passed

The students concluded that pea plants grew better in compost than did melon plants. Which of the following best

addresses the validity of the conclusion made by the students?

a. The conclusion is valid because the independent and dependent variables in the experiment were controlled.

b. The conclusion is valid because the experimental design included a large sample size.

c. The conclusion is invalid because other variables in the experiment affected the results.

d. The conclusion is invalid because the units of measurement were not given in the experimental design.

In order for the conclusion to be valid, all the independent variables should have been controlled. In this case, the independent variables such as the amount of water received by the plants, nutrient requirement of the plants, etc. were not controlled leading to fungal attack on the melon plant.

Hence, the conclusion is invalid because other variables that ought to have been controlled affected the experimental outcome.

The correct option is c.