1. A computer-implemented method for detecting click fraud, the method comprising: extracting activity data from an event database; generating statistical information of unpaid referrals relative to paid referrals for a reference data set; generating statistical information for a data set under analysis; comparing the statistical information for the reference data set and the statistical information for the data set under analysis; and assessing whether or not click fraud is present based at least partly on the comparison; wherein said extracting, generating, comparing, and assessing are performed by one or more computing devices; wherein generating statistical information of unpaid referrals relative to paid referrals for the reference data set comprises computing ratios of unpaid referrals to paid referrals and computing at least one of standard deviation, skewness, or kurtosis of the computed ratios for the reference data set; wherein generating statistical information for the data set under analysis comprises computing ratios of unpaid referrals to paid referrals and computing at least the one of standard deviation, skewness, or kurtosis of the computed ratios for the data set under analysis; wherein the data set under analysis comprises data collected over a shorter period of time than data for the reference data set.