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30 CHAPTER 2 Experimental research

2.2.2 DEPENDENT AND INDEPENDENT VARIABLES
A well-defined hypothesis clearly states the dependent and independent variables of
the study. Independent variables refer to the factors that the researchers are interested
in studying or the possible “cause” of the change in the dependent variable. The term
“independent” is used to suggest that the variable is independent of a participant's
behavior. Dependent variables refer to the outcome or effect that the researchers are
interested in. The term “dependent” is used to suggest that the variable is dependent
on a participant's behavior or the changes in the independent variables. In experi-
ments, the primary interest of researchers is to study the relationship between de-
pendent variables and independent variables. More specifically, the researcher wants
to find out whether and how changes in independent variables induce changes in
dependent variables.
A useful rule of thumb to differentiate dependent variables from independent
variables is that independent variables are usually the treatments or conditions that
the researchers can control while dependent variables are usually the outcomes that
the researchers need to measure (Oehlert, 2000). For example, consider the null hy-
pothesis proposed in the research case in Section 2.2.1:

There is no difference between the pull-down menu and the pop-up menu in the
time spent locating pages.
The independent variable is the type of menu (pull-down or pop-up). The de-
pendent variable is the time spent in locating web pages. During the experiment,
the researchers have full control over the types of menu with which each partici-
pant interacts by randomly assigning each participant to an experimental condition.
In contrast, “time” is highly dependent on individual behavioral factors that the re-
searchers cannot fully control. Some participants will be faster than others due to a
number of factors, such as the type of menu, previous computer experience, physical
capabilities, reading speed, and so on. The researchers need to accurately measure
the time that each participant spends in locating pages and to relate the results to the
independent variable in order to make a direct comparison between the two types of
menu design.

2.2.3 TYPICAL INDEPENDENT VARIABLES IN HCI RESEARCH
Independent variables are closely related to the specific research field. It is obvious
that the factors frequently investigated in medical science are drastically different
from those examined in physics or astronomy. In the HCI field, independent vari-
ables are usually related to technologies, users, and the context in which the technol-
ogy is used. Typical independent variables that relate to technology include:
• different types of technology or devices, such as typing versus speech-based
dictation, mouse versus joystick, touch pad, and other pointing devices;
• different types of design, such as pull-down menu versus pop-up menu, font
sizes, contrast, background colors, and website architecture.