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# What are the key differences between incidence and prevalence

What are the key differences between incidence and prevalence? (2 points)

Incidence measures the occurrence of new disease, while prevalence measures the existence of current disease. Incidence looks at the transition from health to disease, whereas prevalence looks at the period of time that a person lives with a disease. Incidence is the occurrence of new cases of disease that develop in a candidate (“at risk”) population over a specified time period. When measuring incidence, the members of the at risk population are followed until they develop the disease. Prevalence is the proportion of the total population that is diseased. Incidence is most useful in research on the causes, prevention, and treatments of a disease, versus prevalence, which is more useful in estimating the needs of medical facilities and allocating resources for treating people who already have a disease.

What are the key differences between cumulative incidence and incidence rate? (2 points)

Cumulative incidence is the proportion of a candidate population that becomes diseased over a specified period of time. The numerator is new cases and the denominator is the population at risk, and the possible values range from 0 to 1. Cumulative incidence can be seen as the average risk of getting a disease over a certain period of time. Cumulative incidence depends on the length of time and is higher over a long period of time versus a short period of time. Cumulative incidence is mainly used in fixed populations. Incidence rate is the occurrence of new cases of disease that arise during person-time of observation. The numerator is new cases and the denominator is the person-time at risk, and the possible values range from 0 to infinity. Incidence rate is a true rate and is measured in 1/time or t-1. Person-time only counts the time until the individual is diagnosed with the disease of interest and is not dependent on a specified time period. Person-time is only added while the individual is followed and stops when the individual dies or is lost to follow-up. Incidence rate can be calculated for both fixed or dynamic populations, but is extremely useful as a measure of the transition from health to disease in dynamic populations, because it looks at population changes.

QUESTIONS 3-7: For each of the fractions shown below, indicate whether it is an incidence rate OR cumulative incidence or prevalence.

1. Incidence rate
2. Cumulative incidence

3. Prevalence

__A___ 3. number of women in Framingham Study who have died

through last year from heart disease_____________

number of person-years contributed through last year

by women enrolled in Framingham Study

__C__ 4. number of women in town of Framingham who reported having heart disease

in recent health survey_______________________________

estimated number of women residents of Framingham at the time of survey

__B___ 5. number of women in Framingham Study newly diagnosed

__________with heart disease last year_________________

number of women in Framingham Study without heart disease

at beginning of same year

__B___ 6. number of women in State A newly diagnosed with heart disease in 2010

estimated number of women living in State A in 2010

__C___ 7. estimated number of women smokers in State A according to 2010

____________Behavioral Risk Factor Survey________________

estimated number of women living in State A in 2010

QUESTIONS 8-10: Indicate how each of the following conditions influence the prevalence of a disease in a population. For each scenario, assume that no other changes occur. Your choices are:

1. Increases prevalence
2. Decreases prevalence

__A___ 8. A new treatment is developed that prevents death but does not produce recovery from a disease

__A___ 9. A new measure is developed that prevents new cases of disease from occurring

__B___ 10. There is immigration of a large number of healthy people into the population.

The graph below shows the trends in incidence and prevalence for chronic disease Q over a 50-year period. Which of the following interpretations is consistent with the graph below? Indicate as many as could logically be correct.

1. The disease may be becoming more chronic with lower case-fatality rate;
2. The disease may be becoming more rapidly fatal (i.e., it kills patients sooner than before);
3. The disease may be becoming shorter in duration due to better medical treatment;
4. The disease may be becoming rarer due to better preventive public health programs.

QUESTIONS 12-17: Use the following scenario to answer questions 12-17. You must show your work to get credit.

In January 2000 you began a one-year study of tuberculosis (TB) in a subsidized housing community in the Lower East Side of New York City. You enrolled 500 residents in your study and checked on their TB status on a monthly basis. At the start of your study on January 1st, you screened all 500 residents. Upon screening, you found that 20 of the healthy residents were immigrants who were vaccinated for TB and so were not at risk. Another 30 residents already had existing cases of TB on January 1st. On February 1st, 5 residents developed TB. On April 1st, 5 more residents developed TB. On June 1st, 10 healthy residents moved away from New York City were lost to follow-up. On July 1st, 10 of the residents who had existing TB on January 1st died from their disease. The study ended on December 31, 2000. Assume that once a person gets TB, they have it for the duration of the study, and assume that all remaining residents stayed healthy and were not lost to follow-up.

Is the subsidized housing community in the Lower East Side of New York City a dynamic or fixed population? Briefly explain the rationale for your answer.

What was the prevalence of TB in the screened community on January 1st?

What was the prevalence of TB on June 30th? Note: Do not include those that were lost to follow-up because they are no longer “in” the population. They must be “in” the population to be counted in the denominator.

What was the cumulative incidence of TB over the year? Note: Be sure to exclude from the denominator those that were not “at risk.” Remember the assumptions that existing cases have TB for the duration. So, an existing case cannot be a new case, thus they are not at risk.

Suppose that you wanted to calculate the incidence rate of TB in the study population. Calculate the amount of person-time that would go in the denominator of this incidence rate. Be sure to show your work. Note that the 20 vaccinated residents and the 30 prevalent cases did not contribute any person- time because they were not at-risk.

Express the answer in person-months. Hint: You must determine the number of months that each person contributed and add them all together. Let me help you get started… (5 people contributed 1 month each so (5*1mo) +

Extra credit (1 point): Calculate the incidence rate of TB over the one-year period and express answer in person-months.

What was the case-fatality rate among residents with TB over the course of the year?

An outbreak of salmonellosis occurred after a department luncheon, which was attended by 485 faculty and staff. Assume everyone ate the same food items. Sixty-five people had fever and diarrhea; five of these people were severely affected. Subsequent laboratory tests on everyone who attended the luncheon revealed an additional 72 cases. What was the attack rate of salmonellosis? You must show your work to get credit.

QUESTIONS 19-23: Use the following scenario to answer questions 19-23. You must show the numerator and denominator to get credit.

In 2009, a total of 15,555 homicide deaths occurred among males and 4,753 homicide deaths occurred among females. The estimated 2009 midyear populations for males and females were 139,813,000 and 144,984,000, respectively.

Calculate the homicide-related death rates for males per 100,000

Calculate the homicide-related death rates for females per 100,000.

What type(s) of mortality rates did you calculate in Questions 19 and 20?

Calculate the ratio of homicide-mortality rates for males compared to females.

Interpret the rates you calculated in Question 20 as if you were presenting information to a policymaker.

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