(3) Gentzkow M and Shapiro JM, Preschool Television Viewing and Adolescent Test Scores: Historical Evidence From The Coleman Study. Quarterly Journal of Economics, 123: 279-323 (2008).
qje.oxfordjournals.org/content/123/1/279.short
(a) methodology (two consecutive paragraphs):
“In this paper, we identify the effect of preschool exposure to television on adolescent cognitive skills by exploiting variation in the timing of television’s introduction to US cities. Most cities first received television between the early 1940s and the mid-1950s. The exact timing was affected by a number of exogenous events, most notably a four-year freeze on licensing prompted by problems with the allocation of broadcast spectrum across cities. Once it was introduced, television was adopted rapidly by families with children. Survey evidence suggests that young children who had television in their homes during this period watched as much as three and a half hours per day, and contemporary time-use studies show reductions in a wide range of alternative activities, including sleep, homework, and outdoor play. Evidence on television ownership suggests that the diffusion of television was broad-based, reaching families in many different socioeconomic strata. Together, these facts create a promising laboratory in which to study the effects of television on children.” at 280
“To conduct our analysis, we use data from a 1965 survey of American schools and school children, commonly referred to as the Coleman Study. The data include standardized test scores of over 300,000 students who were in grades 6, 9, and 12 in 1965. These students were born between 1948 and 1954, just as television was expanding throughout the United States. Because television entered different US markets at different times, students were exposed to varying amounts of television as preschoolers. Students in our sample range from those who had television in their local areas throughout their lives (for example, sixth graders whose areas got television between 1945 and 1954) to those whose areas only began receiving broadcasts after they reached age 6 (twelfth graders whose areas got television in 1954). Because the Coleman sample includes students of different ages within the same television market, we can identify the effects of television by comparing test scores across cohorts within a given area. This differences-in-differences approach allows us to estimate the effect of preschool television exposure on adolescent test scores, while holding constant fixed characteristics of a locale that affect test scores and might also be correlated with the timing of television introduction.” at 280-281
(b) result
(i) Table V Heterogeneity in the Effects of Preschool Television Exposure on Adolescent Test Scores was at 310, whose discussion started at 309.
(ii) Legend of Table V explained the figures in parentheses were "standard errors"--or standard deviations.
(iii) How to read Table V? At 309:
“The first two columns repeat our basic 2SLS specification for students whose mothers do and do not have a high school education.[36] The estimated effect of a year of television exposure on the average test score is 0.04 for students whose mothers have less than a high school education, and 0.01 for students whose mothers have a high school degree. A similar pattern is present for individual test scores.” at 309 (Footnote 36 at 309 stated, "We obtain similar results using father's education to split the sample rather than mother's education.")
“The next two columns compare households where English was and was not the primary language. The estimated effects of television on verbal, reading, and general knowledge scores for students in non-English-speaking households are positive and nontrivial in magnitude. For the sample of students whose family members primarily speak English, the point estimates are still positive but are much smaller.” at 309
“These findings provide support for the hypothesis that children whose home environments were more conducive to learning were more negatively impacted by television.” at 311
(iv) Pay heed to, in quotation 2, the key word “nontrivial”--which is equivalent to “statistically significant.” In Statistics, 0.05 tends to be the magic number whether two variables correlate: they most likely do if the number is more than 0.05 (meaning that only once out of 20 times is a fluke, by coincidence). In other words, the numbers in quotation 1 (0.1 and 0.4) were statistically INSIGNIFICANT. Early television exposure and later grades were again statistically significant for nonwhite students (0.0526 +/- 0.0489)--yet insignificant for white students (0.0026 +/- 0.0262)
(c) discussion
“As discussed in the introduction, there are important caveats to these results. First, our data speak only to the early childhood television on academic achievement in adolescence. They do not provide evidence on contemporaneous effects, nor do they provide direct evidence on the effects of television on older children [post-adolescence, that is].” at 313
“Finally, a large number of well-known educational television programs have been introduced since our sample period, many of which hage been linked to improvements in early childhood development." at 314
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发表于 4-19-2014 12:09:43
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