SCORE Mathematics
Definition
A lurking variable is a variable that has an important effect on the relationship among the variables in a study but is not included among the variables studied. GRAPHIC
Example
The following joke helps to illustrate lurking variable:
There's this guy who's going to clean the windows of a mental asylum. A patient follows him shouts to him "I gotta secret, I gotta secret...", he ignores the patient. Again the patient follows him, but he ignores his cries. By the time he's nearly finished the building, he's really curious about what the patients secret is, so he decides to ask the patient. The patient pulls a matchbox out of his pocket, opens it and puts it on a table. Out crawls this little spider. The patient says "spider go left", and the spider walks to it's left a bit. Then he says "spider go right", the spider walks to its right a little bit. He says "spider turn around, walk forward then go right", and sure enough the spider turns around, walks forward, and then goes right a bit. The window cleaner is amazed "Wow! He says, that's amazing!", "No, that's not my secret says the patient, watch". He picks up the spider in his hand and pulls all its legs off then puts it back on the table. "Spider go right", the spider doesn't move, "spider go Left", the spider doesn't move, "Spider turn around" again the spider doesn't move. "There!" he says, "that's my secret, if you pull all a spiders legs off they go deaf....................
Case Study
In a regression to predict fuel consumption (measured in Gallons/100 miles -- see the related story "Measuring Fuel Efficiency") from the weight and drive ratio of cars, the Buick Estate Wagon shows up as an outlier. Remarkably enough, even though the Buick Estate Wagon has high fuel consumption when compared with other vehicles, its predicted fuel consumption per 100 miles, as predicted from our regression model, is greater than its observed fuel consumption. An interesting question arises from this example: "Why does the Buick Estate Wagon consume less fuel than we expect?"
The answer is not available in the original data, but was discovered through some additional research on the Buick Estate Wagon. These data were collected by Consumer's Union on a test track (rather than using the EPA test values for fuel efficiency) following the manufacturer's recommendations for each car's maintenance. Additional research revealed that starting with this model year, Buick recommended a higher tire inflation pressure for the Buick Estate Wagon. The recommended inflation pressure level was higher than the level for other cars in the survey. Harder tires present less rolling resistance and improve gas mileage; therefore, the Buick Estate Wagon outperformed our expectations based on our regression model, which did not account for tire inflation pressure. In our model Tire Pressure is a lurking variable, variable that seems to help in predicting gas mileage but is not included in the model.