A more precise way of summarizing the data is the use of effect size (Wolf, 1986; Light and Pillemer, 1984). While measures of statistical significance tell us the chances of obtaining a given result by chance, measures of effect size tell us the impact of the experimental treatment. Two common measures of effect size are the correlation coefficient (r) and "d", where d = (mean of the experimental group-mean of control group) / (pooled standard deviation). (Some analysts use the standard deviation of the control group, rather than the pooled standard deviation; see Wolf, 1986, for discussion; d and r are related to each other by a simple algebraic formula; see Wolf, 1986: p. 35.)

Table 7. Effect sizes for the three experiments
Study Subjects Duration Measure Results Effect size
d r n
1 Four-year college One semester Cloze ER > Trad. 0.702 0.34 40
2a Four-year college One year Cloze ER > Trad. 1.11 0.49 79
2b Two-year college One year Cloze ER > Trad. 1.47 0.60 49
3 Four-year college One year
(a) wrote resp. in Japanese Cloze ER = Cloze 0.244 0.12 78
RC ER > Cloze 0.609 0.30 73
(b) wrote resp. in English Cloze ER > Cloze 0.630 0.30 74
RC ER > Cloze 0.480 0.24 73
ER = extensive reading;
Close = traditional instruction with emphasis on cloze exercises.

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