"Student's" t Test – Interactive tutorial


Random (experimental) error

Let's start with data from a hypothetical study (i.e., these are not real data). Similar data were used in the interactive tutorial on the fundamentals of graphing. You may find it worthwhile to complete the graphing tutorial first, as it includes basic graphing and the rationale behind the standard deviation and how to represent error.

The investigator wished to know the typical growth rates for seedlings of several species of deciduous trees. She planted seeds in a plot area large enough so that individual plants did not complete for light, space, or nutrients. Despite all efforts to use seeds of equal quality and to maintain the same conditions for all plants, individual seedlings varied in height at any given time. We refer to this kind of variation as random error. Most experimental studies are subject to random error, due to factors that cannot be controlled by the investigator. Random error introduces uncertainty into the outcome of an experiment.

The data below represent the heights (cm) of individual plants that germinated from seeds planted three months previously. The investigator measured the first 12 plants in each row and entered the data in that order.

Quercus rubra  6.4 7.4 10.4 8.5 9.2 9.5 11.8 11.8 9.3 12.5 13.7 17
Quercus alba 11.4 9.5 5.8 9.2 11.8 7.9 7.9 7.8 5.3 7.5 7.1 6.6

We want to know which species grew faster under these conditions. Now here is your first question. How can you convert the data to permit you to compare them? Enter a word or phrase below.