Objectives

• To discuss threats to validity for the two articles

• To review for the midterm examination

Threats to Validity

• Smoking prevention paper (Cameron, et al)

• Growth retardation paper (McDermott, et al)

Figure 2. Stroke mortality rates by birth cohort and age group in females.

From:   Medrano: Stroke, Volume 28(1).January 1997.40-44

Discussion• Age effect: Mortality increased markedly with age in all

cohorts for men and women alike, with rates being somewhat higher in men, especially in the younger generations. For any given age group, mortality was observed to vary with cohort, increasing in the older cohorts and decreasing in the more recent ones. The negative slope was less evident for men, the younger age groups, and the more recent cohorts.

• A period effect: was also to be observed, since mortality peaked for all age groups in consecutive cohorts, meaning that the downward shift occurred in the same period for all birth cohorts and age groups. From age and year of birth, it can be deduced that this period must have been circa 1974 (1972-1976).

Table 1. Age-specific rates (per 100000) for schizophrenia.

From:   Takei: Psychol Med, Volume 26(5).September 1996.963-974

Age--period--cohort Analysis of the Incidence of Schizophrenia in Scotland Takei,

Noriyoshi; Lewis, Glyn; Sham, Pak C.; Murray, Robin M. Psychological Medicine Volume 26(5)  September 1996  pp 963-974

Discussion

• Cohort effect is present

• Age effect is present

Rates

• a.      incidence density

• b.      cumulative incidence

• c.      point prevalence

• d.      period prevalence

• e. odds for the disease of interest

Study Design

• a.       case-cohort• b.      case-control• c.       case-control study within a defined cohort• d.      nested case control• e.       cohort• f.        cross-sectional• g.       ecologic• h.       experimental

Sun exposure and melanoma• A random-digit telephone survey of 580 person

was conducted to examine the association of sun exposure and skin melanoma

• Sun exposure was classified as low, medium and high using a standardized algorithm

• Persons who had skin melanoma were compared to those who did not for sun exposure?

• What type of study is this?• What other confounders should be considered?

The association between sun exposure and skin melanoma

Low exposure

Medium

Exposure

High exposure

Skin melanoma

20 20 40

No skin melanoma

200 100 100

What is the prevalence of skin melanoma?What is the prevalence of high sun exposure?

What is the odds of having med/high exposure given that you Have skin melanoma compared to low exposure?

Collapsing Categories

Skin Melanoma

No Skin Melanoma

Low exposure 20 200

Med/High exposure

60 200

ACL tear and gender

Women

(n=1000)

Men

(n=1000)

ACL tear 40 80

No ACL tear 960 920

What is the odds of being a women given you haveAn ACL tear compared to the odds of being a man?

Crude and stratified Odds Ratios

Crude ACL No ACLwoman 70 960 1000

man 80 920 1000

Plays SportsACL No ACL

woman 50 950 1000man 20 980 1000

Doesn't play sportsACL No ACL

woman 20 980 1000man 60 940 1000