Healthcare Statistics
Helpm31990
9781284108217_CH03_SLID.pptChapter 3
Quantifying the Extent of Disease
Learning Objectives
- Define and differentiate prevalence and incidence
- Select, compute, and interpret the appropriate measure to compare the extent of disease between groups
- Compare and contrast relative risks, risk differences, and odds ratios
- Compute and interpret relative risks, risk differences, and odds ratios
Critical Components of RCT
- Randomization
- Control group – ethical issues
- Monitoring
- Interim analysis
- Data and safety monitoring board
- Data management
- Reporting
Prevalence
- Proportion of participants with disease at a particular point in time
Example 3.1.
Computing Prevalence
Prevalence of CVD = 379/3799 = 0.0998 = 9.98%
Prevalence of CVD in Men = 244/1792 = 0.1362 = 13.62%
Prevalence of CVD in Women = 135/2007 = 0.0673 = 6.73%
Example: H1N1 Outbreak
- H1N1 outbreak first noticed in Mexico.
- Large outbreak early on in La Gloria—a small village outside of Mexico City
- Studied extensively in the first report on H1N1 (Fraser, Donelly, et al. “Pandemic potential of a strain of Influenza (H1N1): Early findings,” Science Express, 11 May 2009.)
- Important questions
- Who is most likely to be impacted?
- What are characteristics of people commonly impacted?
Data on H1N1 outbreak in La Gloria, Mexico: n = 1575 villagers (out of 2155) were surveyed to determine if they had influenza-like illness (ILI) between 2/15/09 and 4/27/09.
Computing Prevalence (1 of 2)
| Age | No ILI | ILI | Total |
| ≤ 44 years | 703 | 522 | 1225 |
| > 44 years | 256 | 94 | 350 |
| Total | 959 | 616 | 1575 |
Computing Prevalence (2 of 2)
Prevalence of ILI = 616/1575 = 0.3911 = 39.11%
Prevalence of ILI in ≤ 44 = 522/1225 = 0.4261 = 42.61%
Prevalence of ILI in > 44 = 94/350 = 0.2686 = 26.86%
| Age | No ILI | ILI | Total |
| ≤ 44 years | 703 | 522 | 1225 |
| > 44 years | 256 | 94 | 350 |
| Total | 959 | 616 | 1575 |
Incidence
- Likelihood of developing disease among persons free of disease who are at risk of developing disease
Computing Incidence
- Cumulative incidence requires complete follow-up on all participants.
- Person-time data is used to take full advantage of available information in incidence rate.
- Incidence rate often expressed as an integer per multiple of participants over a specified time.
Incidence of CVD?
Incidence Rate
Incidence of CVD
Incidence = 2/(10 + 9 + 3 + 10 + 5) = 2/37
= 0.054
5.4 per 100 person-years
Example 3.2.
Computing Incidence
Incidence Rate of CVD in Men = 190/9984 = 0.01903
= 190 per 10,000 person-years
Incidence Rate of CVD in Women = 119/12153 = 0.00979
= 98 per 10,000 person-years
| Develop CVD | Total Follow-Up Time (years) | |
| Men | 190 | 9984 |
| Women | 119 | 12153 |
| Total | 309 | 22137 |
Computing Incidence
Incidence Rate of ILI in ≤ 44 = 522/20,064 = 0.0260
= 260 per 10,000 person-years
Incidence Rate of ILI in > 44 = 94/3514 = 0.0268
= 268 per 10,000 person-years
| Developed ILI | Total Follow-Up Time (years) | |
| ≤ 44 years | 522 | 20,064 |
| > 44 years | 94 | 3,514 |
| Total | 616 | 23,578 |
- Risk difference (excess risk)
Comparing Extent of Disease
Between Groups (1 of 2)
Comparing Extent of Disease
Between Groups (2 of 2)
- Risk difference of prevalent CVD in smokers versus nonsmokers
= 81/744 – 298/3055 = 0.1089 – 0.0975 = 0.0114
Population Attributable Risk of CVD in Smokers vs. Nonsmokers
= (0.0998 – 0.0975) / 0.0998 = 0.023 = 2.3%
- Risk difference of history of ILI in males and females in La Gloria
= 356/798 – 260/777 = 0.4461 – 0.3346 = 0.1115
Comparing Extent of Disease
Between Groups (1 of 7)
| No ILI | ILI | Total | |
| Males | 517 | 260 | 777 |
| Females | 442 | 356 | 798 |
| Total | 959 | 616 | 1575 |
- Relative risk
Comparing Extent of Disease
Between Groups (2 of 7)
Comparing Extent of Disease
Between Groups (3 of 7)
- Relative risk of CVD in smokers versus nonsmokers
= 0.1089/0.0975 = 1.12
- Relative risk of ILI in females versus males
= 0.4461/0.3346 = 1.33
Comparing Extent of Disease
Between Groups (4 of 7)
| No ILI | ILI | Total | |
| Males | 517 | 260 | 777 |
| Females | 442 | 356 | 798 |
| 959 | 616 | 1575 |
- Odds ratio
Comparing Extent of Disease
Between Groups (5 of 7)
- Odds ratio of CVD in hypertensives versus hypertensives
Comparing Extent of Disease
Between Groups (6 of 7)
Comparing Extent of Disease
Between Groups (7 of 7)
- Odds ratio of ILI in younger group versus older group
| Age | No ILI | ILI | Total |
| ≤ 44 years | 703 | 522 | 1225 |
| > 44 years | 256 | 94 | 350 |
| Total | 959 | 616 | 1575 |
Relative Risks and Odds Ratios
- Not possible to estimate relative risk in case-control studies
- Possible to estimate odds ratio because of its invariance property
- Case-control study to assess association between smoking and cancer
Invariance Property of Odds Ratio
(1 of 2)
Invariance Property of Odds Ratio
(2 of 2)
Odds ratio for cancer in smokers versus nonsmokers
= (40/29) / (10/21) = 2.90
Odds of smoking in patients with cancer versus not
= (40/10) / (29/21) = 2.90(!)
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