The proportion of apprentices and trainees completing their contract of training has received considerable attention in recent years. To support public discussion, the National Centre for Vocational Education Research (NCVER) has been publishing tabulations of both completion and attrition rates. Up until recently, the approach taken was to identify a commencing cohort and follow it over time. Currently, NCVER publishes completion and attrition rates which are calculated from tabulations of completions and cancellations/withdrawals within particular quarters. This technical paper details a methodological approach which constructs cross-sectional estimates of completion and attrition rates for the latest commencing cohorts. These reflect what completion and attrition rates would be if the patterns observed in a particular quarter remain unchanged for the life of the commencing cohort in that quarter. Throughout the paper, the authors use a simple and contrived example to illustrate the model, along with the formal mathematical notation. Finally, they apply the model to cohorts commencing in the December quarters of 2007 to 2009. (Contains 5 tables, 3 figures and 2 footnotes.) [For the accompanying report, "Experimental Completion and Attrition Rates for Latest Commencing Apprentices and Trainees", see ED510878.]