1. مطالعات کوهورت Cohort Studies

2. Cohort studies
Involves comparing disease incidence over time between groups (cohorts) found to differ on their exposure to a factor of interest. Two types: • prospective • retrospective prospective cohort study begins with the selection of two groups of non-diseased animals: • exposed to a factor postulated to cause a disease • unexposed to a factor postulated to cause a disease retrospective cohort study starts when all of the disease cases have been identified. History of each study participant is carefully evaluated for evidence of exposure to the agent under investigation

3. Schematic diagram of a prospective and retrospective cohort study

4. Advantages of Cohort studies
• You can examine multiple outcomes for a single exposure, • Are very useful in examining rare exposures • Provide estimates of the absolute incidence of disease in exposed and non-exposed individuals, • Temporal sequence is logical Exposure status is recorded before disease has been identified • Provides unambiguous information about whether exposure preceded disease. • Well-suited for studying rare exposures. • Relative number of exposed and non-exposed persons in the study need not necessarily reflect true exposure prevalence in the population at large. Subjects are monitored over time for disease occurrence,

5. Disadvantages of Cohort studies
• Prospective cohort studies require a long follow-up period. Framingham study, 5,000 residents of framingham, massachusetts, since the early 1950’s to establish the rates and risk factors for heart disease • in the case of rare diseases large groups are necessary. • because of following forward in time likely of logistical problems, • Losses to follow-up can become an important problem. • Often quite expensive to run.

6. جدول اتفاقات Contingency table
بیمار
سالم
جمع
دارای تماس a  b 
a+b
بدون تماس c  d
C+d
a+c 
b+d
A+b+c+d

7. مقادیر در کوهورت a/a+b = Attack Rate for exposed
c/c+d = Attack Rate for non – exposed
Attack Rate for exposed/ Attack Rate for non – exposed = Relative risk (RR)
a/(a + b) / c/(c + d) = Relative risk (RR)

8. تفسیر مقادیر کوهورت
اگر خطر نسبی برابر 1 شود یعنی ارتباطی بین تماس exposure و بیماری یا عارضه وجود ندارد.
اگر خطر نسبی بزرگتر از 1 باشد یعنی تماس با بیماری ارتباط دارد و تماس یک عامل خطرزا یا عامل خطر Risk factor است.
اگر خطر نسبی کوچکتر از 1 باشد یعنی تماس باعث محافظت از بیماری یا عارضه است یعنی تماس یک عامل محافظت Protective factor است.
Determinants

9. محاسبه خطر نسبی برای روستاهای واکسن خورده در فاز 22 واکسیناسیون همزمان تب برفکی (25 شهریور تا 30 مهر ) و ابتلا به بیماری از مورخ 5 آبان تا 24 آذر (5 روز بعد از پايان واكسيناسيون همگاني فاز 22)
تعداد روستا با همه گيري مثبت
تعداد روستا با همه گيري منفي
جمع
Attack rate
تعداد روستاي واكسن خورده
158 
23,262  
23,240
ARv = 158/23,240 =
تعداد روستاي واكسن نخورده
49 
34,177
34,226
ARnv = 49/34226 =
بنابراين خطر نسبي Relative Risk (RR) براي روستاهاي واكسن خورده برابر:
RR = /0.0014= 4.78

10. • Strength of association – the relationship must be clear,
In general, five criteria must be met to establish a cause and effect relationship
• Strength of association – the relationship must be clear,
• Consistency – observation of the association must be repeatable in different populations at different times.
• Temporality – the cause must precede the effect.
• plausiblity – the explanation must make sense biologically.
• Biological gradient – there must be a dose – response relationship.

11. Outbreak of gastroenteritis following a church supper in Oswego
Of 80 persons who attended the supper, 75 were interviewed. Forty-six persons met the case definition. Attack rates for those who did and did not eat each of 14 items are presented in the table.

13. The relative risk is calculated as 79.6 / 14.3, or 5.6.
Attack rate by consumption of vanilla ice cream, Oswego, New York, April 1940
The relative risk is calculated as 79.6 / 14.3, or 5.6.

14. مسمومیت با زئولیت در گرگان
آنها که در آب خورده بودند.
آنها که در غذا خورده بودند.
آنها که نخورده بودند.

15. Statistical significance
first step assume that the exposure is not related to disease , known as the null hypothesis.
if the null hypothesis proves to be implausible, alternative hypothesis,
compute a measure of association, such as a relative risk or odds ratio.
calculate a chi-square or other statistical test.
Consider p-value or probability.
A very small p-value: very unlikely to observe such an association if the null hypothesis is true.
p-value smaller than some cutoff, such as 5%,accept alternative hypothesis

16. For a two-by-two table, the chi-square formula is:

18. تفسیر Chi - square
Once you have a value for chi-square, you look up its corresponding p-value in a table of chi-squares, such as Table 6.6. Since a two-by-two table has 1 degree of freedom, a chi-square larger than 3.84 corresponds to a p-value smaller than This means that if you have planned to reject the null hypothesis when the p-value is less than 0.05, you can do so if your value for chi-square is greater than 3.84.