From the case study 5.2 (pg78), Grover’s Corner DOT cannot easily determine the external validity of the data collected due to various reasons which are mainly brought out by the change in the method of determining the rail users. First,people might have changed the means of transport after the change in counting method. The system do not account for the probable decline or increase in commuters using the rail as a result of the new method of getting exact numbers instead of estimates.The study also do not account for any external change in other available alternatives. Presence of other means of transport could also pose a challenge to the consistency of results; other available means may have been made better and hence attracted more people who used to travel in rail. Also they could have worsened, making more people opt to travel with rail. These factors make it challenging to determine the external validity.

Timing could as well have influenced the results. The research does not leave out the chances of presence of unusual activities during the period. There are chances of bias on the number of people present depending on timing of the events; there could have been more people in case the research was done on a peak and also less number in case it was done on a low season, this would not count as the best sample to represent the results without these considerations.

Factors That Might Limit External Validity of Findings

Lack of uniformity in the study- it is evidently clear that the method used for the whole period under study is not uniform since there was a change from spot check where only random values were obtained to the new method where exact number count is taken.Inadequate information about other influencing factors- in order to make sound generalizations, all information should be provided about other factors that could have influenced the research. The research should have provided for any change in other means of transport so as to draw conclusions on their effect on the sector.