Results of an experiment may suggest that the independent and dependent variables are related. The relationship between variables—the correlation, can be positive, where both variables increase or decrease together. Or negative, one increases and the other decreases. Also, there may be no relationship between the variables. To determine if an apparent correlation reflects a direct cause-and-effect association—a causal relationship, additional control experiments must be performed. Consider a study to determine the cause of tail loss in five gecko populations. If a researcher finds a decrease in the number of geckos without tails as the number of parasitic crow ticks increases, this result would be a negative correlation. And one could conclude that the crow parasite is not directly causing the tail loss in geckos. But, if the researcher had counted the number of crows at each location, he may have found a positive correlation between the number of crows and the number of tailless geckos. And after examining the crow's stomach contents, he would have also found the missing gecko tails. So, the number of crows directly determined the number of geckos that lost tails.