Statistical Methods For Detection Of Answer Copying

Answer copying is the hardest form of cheating to detect. There are two types of statistical methods to detect it: answer-copying statistics (ACS) and person-fit statistics (PFS). In ACS the likelihood of two examinees to give same answer pattern is calculated, the copier is detected if the likelihood is rather small. PFS compares an observed item score pattern with a test model predicted, examinees who give aberrant item pattern are suspected to be the copier. Many other factors besides answer copying can give rise to inflated PFS. ACS is especially presented to detect answer copying and has been broadly used to detect answer copying in SAT, Licensure and certification examinations in America. Many researches were found that the detection rate of ACS was higher than PFS.The research simulated the data in the way of Monte Carlo,and investigated the influences of the test length, sample size, percentage of items copied and ability level of source to the empirical type I error, the detection rate of the ACS and PFS. The results of the simulation study showed that the statistics of K~*, K1, K2 and S1 were able to hold the type I error rates below theαnominal level, S2 and lz were liberal and above the nominalαlevel in most situation,ωcould maintain its type I error on or slightly below the nominalαlevel. Furthermore, the detection rate were enhanced with the increase of the number of items and percentage of items copied. Results further showed that K~*,K1,K2,S1 resulted in higher detection rates for large sample size except lz andω, the detection rate of lz andωwere similar for different sample sizes. When the rate of copying was lower(10%,20%) , the ability level of source hadn't effect on detection rate, but when the rate of copying was higher(30%,40%), the detection rate was improved with the increase of ability level of source. In all, the series of K had lower detection rate compared to other statistics, but K2 was better than K*and K1, the S2 andωshowed significant improvement in detection rate. The detection rate of lz was higher than S1, and lower than K2.