TABLE 1.
A Comparison of Participating and Non-Participating Families in the TBED-C
| Twin/family characteristic | Mean (SD) for participating twins/families | Mean (SD) for nonparticipating twins/families | Cohen’s d effect size |
|---|---|---|---|
| Twin conduct problems | 1.33 (1.23) | 1.39 (1.30) | −0.05 |
| Twin emotional symptoms | 1.80 (1.44) | 1.79 (1.48) | 0.01 |
| Twin hyperactivity | 2.92 (1.96) | 3.08 (1.98) | −0.08 |
| Family income | $72,027 (43,800) | $78,319 (50,300) | −0.13~ |
| % two parent homes | 89.3 (34.0) | 86.0 (36.8) | 0.09 |
| % dizygotic | 51 (50.1) | 55 (50.0) | −0.08 |
| % conceived with fertility meds | 20 (40.2) | 22 (41.2) | −0.05 |
| Number of twin siblings | 2.1 (1.1) | 2.2 (1.0) | −0.10 |
| Paternal years of education | 7.9 (2.6) | 7.9 (2.6) | 0.00 |
| Maternal years of education | 8.7 (2.3) | 8.3 (2.4) | 0.17* |
| Paternal age at assessment (in yrs) | 40.3 (6.3) | 39.7 (6.0) | 0.10 |
| Maternal age at assessment (in yrs) | 38.5 (5.6) | 38.1 (5.5) | 0.07 |
| % of mothers with alcohol problems | 2.0 (14.1) | 1.3 (11.4) | 0.05 |
| % of fathers with alcohol problems | 8.7 (28.3) | 8.0 (27.2) | 0.03 |
| % paternal felony convictions | 5.0 (21.8) | 6.9 (25.4) | −0.01 |
| % maternal felony convictions | 0.4 (6.7) | 3.1 (17.5) | −0.20* |
| Absolute average effect size | .076 |
Note: Twin conduct problems, emotional symptoms, and hyperactivity were assessed using the Strengths and Difficulties Questionnaire (SDQ; Goodman & Scott, 1999). Each SDQ scale has a possible range of 0–10. National norms for mean SDQ conduct problems, emotional symptoms, and hyperactivity among 8–10 year-olds were 1.3, 1.5, and 2.9, means that are quite comparable to those reported here. Similarly according to the 2008–2010 American Community Survey, the mean family income for the state of Michigan is $73,373, an income level that is quite comparable with that seen in our families. Parental education was coded on a scale of 1–12, with 1 indicating the completion of 7th grade and 12 indicating the completion of more than 2 years of graduate school (7 and 8 correspond to the completion of 2 and 3 years of college, respectively).
Indicates that the difference between participating and non-participating families was significant at p < .05. ~ indicates marginally significant difference (p < .10). Please note, however, that none of the means were significantly different across participation status once we Bonferroni-corrected for the number of statistical tests.