The OCHS Approach

1) for 194 subjects, psychiatrist assessment

2) Examine False +ve, False -ve numbers at different cutpoints on parent, teacher questionnaire

SCORE	False +ve	False -ve

7	15	2

8	5	5

9	2	15

PREVALENCE OF DISORDER

Obviously, the prevalence of disorder is very sensitive to the location of the cutpoint.

Parent --- Emotional Problems

Score	N	Prevalence

1	1174	48%

2	274	36%

3	204	9.0%

4	190	8.3%

5	115	5.0%

6	110 4.8%

7	69	3.0%

8	37	1.6%

9	32	1.4%

10	29	1.3%

>10	43

2277

Relation between Parent and Teacher Ratings

"Multitrait – MultiMethod Matrix"

CONDUCT	EMOTIONAL

Parent	Teacher	Parent	Teacher

Parent	1.0	.166	.226	.040

CONDUCT

Teacher	.166	1.0	.025	.380

Parent	.226	.025	1.0	.062

EMOTIONAL

Teacher	.040	.380	.062	1.0

EXPECT	High correlation across traits

Conduct	Emotional

.16	.06

Low Correlation across Methods

Parent	Teacher

.23	.38

Correlation with Intermediate Outcomes

Family Dysfunction	School Problem

PARENT

Conduct	.14	-.09

Emotional	.06	-.05

TEACHER

Conduct	.014	-.08

Emotional	-.015	-.01

MEASURES OF ASSOCIATION

between

RISKS AND OUTCOMES

Example:

Does having no science prerequisites (RISK FACTOR) lead to greater likelihood of having problems in medical school (OUTCOME)

PROBLEMS

yes	no	TOTAL

no	20	130	150

SCIENCE

yes	30	570	600

50	700	750

RISK (Prob|No Science) = 20/150 = 0.133

RISK (Prob|Science) = 30/600 = 0.050

Relative Risk of Problems = .133 / .050 = 2.67

Attributable Risk =

Additional risk of problems attributed to no science prereq

= .133 - .050 = 8.7%

ODDS RATIO versus RELATIVE RISK

PROBLEMS

yes	no	TOTAL

no	20	130	150

SCIENCE

yes	30	570	600

50	700	750

Relative Risk = 2.67 (as before)

ODDS of (Prob| No Science) = 20/(150 - 20) = 0.153

ODDS of (Prob|Science) = 30/(600-30) = .052

ODDS RATIO = .153/.052 = 2.94

WHY do they use ODDS RATIO?

1) In designs where people are selected based on outcome (e.g. case-control studies of Cancer) then typically the "prevalence" of the outcome is 50% (since you select cases and controls equally). The real prevalence is unknown. The ODDS RATIO provides a reasonable approximation

2) They are routinely using "multivariable" methods to examine joint contributions of different variables (Logistic regression). The OR falls easily out of this analysis

Different Approaches to the Contingency Table

SCHOOL PERFORMANCE

Poor	OK

YES	52	131	183

(28%) a	b

On Welfare

c	d

NO	332	2417	2741

(12%)

384	2588

Odds Ratio = a/b = ad /bc = 52 x 2417 / (131 x 332) = 2.89

c/d

= Phi Coefficient = (ad-bc)/ (a+b) (a+c) (b+c) (b + d) = 0.117

Chi - square =	40.24, p<.00001