Thompson (2007) shows, in his own six model specifications and his own concluding caveat, that the Liberty-ship forgetting rate is highly specification-sensitive — not a single number

economicslearning-curveorganizational-forgettingliberty-shipspeter-thompsonshipbuildingwwii-economics

Peter Thompson's re-analysis of the WWII Liberty-ship shipbuilding case fits three learning-curve specifications (loglinear, "accumulation," "replacement"), each both with and without a correction for unobserved product-mix changes, then adds labor-turnover controls to each — six specifications total, on the same dataset, in the same paper. Without the product-mix correction, the loglinear and accumulation specifications each estimate a monthly forgetting rate around 8.4–8.5 percent; the replacement specification returns 5.8 percent. Correcting for product-mix changes lowers the loglinear estimate "by exactly half to 4.2 [percent] per month," and the bounded-learning models "by about 35 percent, yielding estimates of 5.7 percent per month for the accumulation model and 3.6 percent per month for the replacement model." Adding labor-turnover controls collapses the estimate further still — one specification puts the monthly rate "between −1.4 and +2.0 percent" — and Thompson states that turnover data "essentially eliminated all evidence of organizational forgetting" in some specifications, even though "labor turnover appeared to be largely unrelated to productivity changes" itself. Across the six, point estimates span from a negative rate (mild anti-forgetting) to 8.7 percent per month.

Thompson does not treat one of these six as "the" answer and leave the spread implicit. His own conclusion names the instability directly: "It is prudent to close with a note of caution. The fact that we are necessarily dealing with strongly trending data makes the estimated rate of forgetting quite sensitive to model specification, while allowing wide variations in the forgetting rate to account well for variations in the data." He illustrates with his own Table 2 estimate — "a rate of forgetting of 1.6 percent per month, with a 95 percent confidence interval of [0.2 percent, 3.0 percent]" — a range wide enough that, in his own account, the practical implications differ vastly at the two ends.

The contestedness of the Liberty-ship forgetting rate is therefore a finding Thompson states about his own results, not an artifact manufactured later by secondary papers picking single numbers off his tables. It sharpens the picture the vault already had from Benkard's (2000) comparison across industries: forgetting rates vary not only across studies and industries but within a single author's own robustness checks on one dataset.