Apparent-time hypothesis

In sociolinguistics, the apparent-time hypothesis(显象时间假设) states that age-stratified(年龄分层) variation in a linguistic form is often indicative of a change in progress. That is, if in a survey of a population, patterned differences between the speech of individuals 75 years old, 50 years old, and 25 years old may indicate changes that have occurred over the past 50 years. The apparent-time hypothesis depends on several assumptions: first, that a significantly broad sample is taken to be representative of the population; second, that vernacular(本国的、方言的) speech is relatively stable in a given individual once that individual is past adolescence.

J.K. Chambers, writing in 2002, has cited an example of the application of the apparent-time hypothesis. The study, carried out in central Canada, examined the sociolinguistic variable (wh), where the unvoiced labiovelar glide /hw/ loses phonemic status and merges with the corresponding voiced glide /w/. In this study, the oldest subjects seem to indicate a stable period for this variable, both the 70–79 year olds and those over 80 used the voiced variant where the unvoiced was "expected" 38.3 and 37.7% of the time, respectively. Each subsequent younger age cohort (10 years) shows a greater percentage of /w/ usage, with those 20–29 using /w/ 87.6% of the time and the teenagers using it 90.6% of the time. Notice that the deltas between the oldest two groups and between the youngest two groups are relatively small, 0.6% and 3.0%. Between these two extremes the rate of change between the groups is quite high, approximately 10% per age cohort. This pattern can be described as an initial stable period, followed by a period of rapid change, and a tailing off as the change nears completion. This S-curve pattern has been identified as characteristic for many types of linguistic changes.

Not all age-related variation indicates change in progress. It may be an age-graded variation. The applicability of the apparent-time hypothesis should be confirmed by real-time evidence, which actually samples the population over an extended period of time. This is the only true indicator of change in progress. Real-time evidence may come from a longitudinal study of a population or by replicating a study conducted at some relatively distant time and comparing the observations to those previously published.

References
Bailey, Guy (2002). Real and Apparent Time. In J.K. Chambers, Peter Trudgill, & Natalie Schilling-Estes [ed], The Handbook of Language Variation and Change (pp 312-331). Oxford, England: Blackwell.
Chambers, J.K (2002). Patterns of Variation including Change. In J.K. Chambers, Peter Trudgill, & Natalie Schilling-Estes [ed], The Handbook of Language Variation and Change (pp 358-361). Oxford, England: Blackwell.
Eckert, Penelope (1997). Age as a Sociolinguistic Variable. In Florian Coulmas [ed], The Handbook of Sociolinguistics (pp 151-154). Oxford, England: Blackwell.