#2. There are some confounding factors in this study that make it difficult to judge whether changes in the dependent variables were caused by the independent variable (Jackson, 2012; P.229). First, the study does not include a control group, which is the most important element of an experiment. Other confounding factors that may have influenced the outcome include historical effects due to study duration; Additionally, mortality or dropout rate could be a factor as we knew that 50 subjects were selected for the study but only 29 completed the study (42% dropout rate). So it is possible that some of these uncontrolled extraneous variables influenced the outcome of the research (Jackson, 2012; P.229).#4. Internal validity refers to the correctness of research inferences regarding cause and effect while external validity involves the ability to generalize study findings to other contexts (Cozby & Bates, 2012, p. 69). These are important because threats to the internal validity of experimental procedures affect the researcher's ability to draw a valid conclusion “from population data in an experiment” (Creswell, 2014; p. 174). Furthermore, the threat to the external validity of the study will affect the ability to generalize the result of the study “beyond the subjects used for the experiment and beyond the laboratory in which the experiment was conducted” (Jackson, 2012; P.239). #6. Both the within-subjects design and the matched-subjects design used different participants in each condition; but one of the main differences is carry-over effects, that is, participating in changes in one condition makes it difficult to participate in the second condition (Jackson, 2012; P.242). What is the purpose of conducting an experiment? How an experimental design works de… half of the paper… in which participants are matched across conditions on variables that the researcher deems relevant to the research. This type of design shares some commonalities with both between- and within-subject designs. However, in the mood study above, the researcher must try to match the participants (experimental groups/control groups) in the relevant variables (smile; not smile). Since there are different people in each group, this will help minimize test effects and demand characteristics. Because matched-subject experimental designs share the same attributes of between-participants and within-participants designs, it has the same advantages as both. But the weakness of experimental designs with matched subjects is that more subjects will be needed for the research. Furthermore, the loss of one subject due to attrition will affect the entire couple (Jackson, 2012; P.242).