1. Biostatistics and Research Design, #6350
Incidence, Prevalence
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2. Thoughts for the Day: Definition of a Taxpayer:
A person who does not have to pass a civil service exam to work for the government!
Ronald Reagan
“The income tax has made more liars out of the American people than golf has…”
Will Rogers
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3. Why are disease rates and risk factors important?
Describing your patient community
e.g., ethnic mix and disease rates
Designing programs
e.g., screening program for glaucoma if elderly population
e.g., school screenings
Evidence for unusual occurrences - epidemics (e.g., cancer, infectious disease (anthrax))
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4. Incidence: Definitions
The probability that healthy people will develop a disease over a specified period of time
Rate at which new disease occurs in a group of people who are disease-free
Measurement of the rate at which new cases of a disease appear
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5. Incidence numerator = new cases denominator = population at risk
specified period of time, usually one year (time component is IMPORTANT!)
denominator is usually standardized , ,000
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6. Where Incidence Comes From: Prospective Cohort Studies
Beaver Dam Eye Study: 10 yr incidence of ARMD
Orinda Longitudinal Study of Myopia:
Tracked ocular components and RE over time (~3 years)
Reportable Disease Registries: e.g.:
Centers for Disease Control and Prevention;
Morbidity and Mortality Weekly Report (MMWR)
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7. Incidence/Risk of Everyday Events
Handout: Table 6.1, from: Gerstman, BB. Epidemiology Kept Simple.  2003, Wiley-Liss Inc., Hoboken, New Jersey, p126.
Keeps things in perspective!
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8. Incidence: Clinical Example I
What is the incidence of contact lens related corneal ulcers?
Abstract handout
Cheng et al., Incidence of contact-lens-associated microbial keratitis and its related morbidity. Lancet; 1999, 354:181-5.
New cases in 3 month period in 1996
Netherlands
Nation-wide prevalence of CL wear estimated
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9. MK Examples* Acanthamoeba Pseudomonas aeruginosa
* Photos courtesy of Dr. Barry Weissman, UCLA
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10. Incidence: Clinical Example I
Results:
92 cases of microbial keratitis
Annualized Incidence:
1.1/10,000 for daily wear RGPs
3.5/10,000 for daily wear SCLs
20.0/10,000 for extended wear SCLs
Conclusions:
Similar to US figures for 1989 (materials not helping)
Strongly discourage EW
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11. Incidence: Clinical Example II
What is the incidence of contact lens related microbial keratitis in Australia (2008)?
Abstract handout
Stapleton F, et al., The incidence of contact lens-related microbial keratitis in Australia. Ophthalmol. 2008; 115:
New cases in 12 month period in
Australia
Nation-wide usage of CL wear estimated (for risk factors)
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12. Incidence: Clinical Example II
Results:
285 cases of microbial keratitis, 1,798 controls
Annualized Incidence:
1.2/10,000 for daily wear RGPs (vs. 1.1 for I)
1.9/10,000 for daily wear SCLs (vs. 3.5 for I)
19.5/10,000 for extended wear SCLs (vs for I)
11.9/10,000 for daily wear Sil-Hydrogel
25.4/10,000 for extended wear Si--Hydrogel
Conclusions:
Similar to US figures for 1989 (materials not helping)
Strongly discourage EW
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13. Prevalence: Definition
Probability of people having a particular disease at a given time: cross-sectional studies
These are existing cases - influenced by the number of people who have had the disease in the past and the duration of the condition
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14. Prevalence Point prevalence - prevalence at a given point in time
NB: sort of like point estimate like mean
e.g., MGD: How many during senior rotations (student project)?
Period prevalence - prevalence at a point + new cases + recurrences within a given period - most often a year
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15. Prevalence numerator = existing cases
denominator = total population at risk
point or period
denominator is usually standardized often seen per 100,000
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16. Where Prevalence Comes From: Cross-sectional studies, sometimes cohort studies
e.g., Student MGD study
e.g., SEE Dry eye study (DE in elderly in MD)
Framingham Eye Study (glaucoma, diabetic retinopathy, ARMD etc.)
Question generalizability!
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17. '10 - '11

18. Recent Dry Eye Prevalence Figures
Author/ Study
Study
Method
Prevalence
Schaumberg et al., 2003
Cross sectional
Women only
49-89 yrs
USA
Questionnaire
“Dry” if severe Sx or prior Dx of dry eye
n = 36,995
Age-adjusted = 7.8% > age 50
McCarty et al., 1998
Age > 40 yrs
Australia
Clinical Testing
N = 926
7.4% w/two or more signs
Schein et al., 1997
Age > 65
n = 2,420
3.5% w/both Sx and low Schirmer OR high RB
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19. Prevalence of MGD by Decade*
Overall Prevalence, all ages, = 155 MGD /398 total n  39%
n = 61
n = 35
n = 50
n = 43
n = 165
Percent MGD Positive
n = 33
n = 11
Age by Decade
* Hom et al., Optom Vis Sci; 67: , 1990.
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20. Relationships: the water tank analogy
New Cases
New Cases
Old Cases
Incidence (inflow)
Prevalence (water level)
Former Cases
Recovery or death
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21. Comparison of Incidence and Prevalence*
Counts new onsets only
Level is independent of mean duration
Measured as rate or proportion (%)
Reflects likelihood of developing disease over time
Preferred when studying disease etiology
Prevalence
Counts “new” and ‘old” cases
Depends on mean duration of disease
Always a proportion (%)
Reflects likelihood of having disease over time
Preferred when studying health services utilization
* From Gerstman, BB. Epidemiology Kept Simple.  2003, Wiley-Liss, New Jersey, p. 135.
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22. Relationships, Examples
Common cold: low prevalence even though incidence is high-Why?
Short duration
Similarly, bacterial conjunctivitis = low prevalence
Chronic Eye Diseases, e.g., glaucoma:
High prevalence even though relatively low incidence since disease is chronic
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23. Prevalence: Targeting Rates for Populations
Expressed relative to: (all very common):
Age
Gender
Ethnicity
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24. Examples from Literature
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25. The Framingham Eye Study I, Results:
Prevalence: Varies by author & criteria!!!
Similar to dry eye
1 or both eyes positive*:
Cataract  15.5%
Diabetic retinopathy  3.1%
ARMD  8.8%
(P)OAGlaucoma  3.3%
* Kahn, et al., Am J Epidemiol, 106:17, 1977.
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26. Examples from Literature
Major Eye Studies: from: Hatch, S.W. (ed.) Ophthalmic Research and Epidemiology: Evaluation and Application. © 1998; Butter-Worth Heineman, New York; Tables A-5 and A-6.
Demonstrates:
Age and prevalence relationship (Table A-6 especially)
Variation among studies (differing diagnostic criteria)
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27. Note: Terminology
Morbidity = Illness
Mortality = Death
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