# Presentation: 310764

## Public Health Case Study

- Mark Wathen
- Is This Really Okay for Me?A Public Health Statistical Analysis
- CDC’s 2005 Science Ambassador Program

## Case Study Outline

- Public Health
- Surveillance
- Epidemiological Studies
- Calculations
- Prevalence
- Incidence
- Risk Ratio
- Prevention

**Notes:**
This presentation focuses on the public health system through the presentation of a fictional case study.  The majors topics to be discussed are given in this outline.  Definitions and examples of these topics will be presented in the following slides.

## Public Health

- Is the science and art of preventing disease, prolonging life, and promoting health through the organized efforts of society including
- _Monitoring_ disease and other health-related conditions (surveillance); and
- _Identifying_ and _quantifying_ important trends (epidemiology).

**Notes:**
Public health examines the health of the population as a whole.  This entails the monitoring of current health conditions.  This monitoring and collection of data is referred to as surveillance.  Epidemiological studies are implemented to measure and identify if there are trends in the data.  If trends are present, then they are quantified to determine if they are a deviation from normal or simply a random fluctuation in the data.      

## Surveillance

- Is the _monitoring_ of the occurrence of a disease or other health-related condition.
- Active surveillance
- Actively searching for people with a disease or condition.
- Passive surveillance
- Dependent on reporting from external sources.

**Notes:**
Surveillance can be active or passive.  A researcher that goes to a hospital or local clinic looking for people with a certain condition or disease is doing active surveillance.  However, if that same researcher waits for information to be reported by a physician or other medical professional, then the surveillance is considered passive. 

## Epidemiology

- The study of the distribution, determinants, and deterrents of disease or other health-related conditions.
- _Distribution_ is the frequency and patterns in terms of person, place, and time (who, where,  and when).
- _Determinants_ are factors that cause or contribute to a disease or condition.
- _Deterrents_ are factors that prevent or reduce the chance of developing a disease or condition.

**Notes:**
Epidemiology focuses on identifying and quantifying factors that may be effecting health conditions within a population.  For example, a study may need to be conducted if a researcher investigating the number of children born with spina bifida noticed a rapid increase in the number of children affected.  This study would first need to address the who, where, and when questions.  Then the researcher would look at the possible factors that may be causing the increase in cases of children with spina bifida.  

Researches would also be concerned with a gradual increase in health related conditions.  For example, there is concern now with the growing population of people who are obese.  Researchers are looking into the factors that might contribute to the rise in the number of people who are obese.     

## Prevalence

- The proportion of a population that has a disease or health-related condition during a specific time.

**Notes:**
Prevalence quantifies the amount of people with the condition in comparison to the whole population.  Prevalence gives the researcher an idea of the amount of cases occurring right now.  The larger the prevalence, the larger the proportion of the population who are affected.  

## Case Study

- February 2004.
- School nurse reported 39 cases of the flu in the high school.
- Age of students 14–18 years
- Total of 780 students in the school.
- What was the prevalence of the flu for this high school?

**Notes:**
Here is a fictitious case study that is used as an example to work prevalence and risk ratio (risk ratio will be explained in upcoming slides) calculations.  

During the month of February (2004), the school nurse reported 39 cases of the flu in the high school (14 – 18 years of age).  If there are 780 students in the school, what is the prevalence of the flu for this high school? 

The following slide will go through the prevalence calculation. 

## Prevalence Calculation

- Report who, where, and when.
- Population: Students aged 14–18
- Place: High school
- Time: February 2004

**Notes:**
First, it is important to report were the data is coming from.  These are the who, when, and where questions.  To calculate prevalence we divide the number of people affected by the total population.  In this example there are 39 students with the flu and a total population of 780.  39 divided by 780 equals .05.  A value of .05 has little meaning, so it is multiplied by the appropriate factor of 10 until it becomes a whole number.  In this example the factor would be 100.  So, multiplying .05 by 100 gives 5.  This data is then reported as 5 out of 100.  Reporting the data in this manner is more meaningful.  People can relate to 5 out of 100 easier than .05.

## Case Study—Continued

- Results 5 out of 100 students had the flu.
- Monitor the situation during the month of March (2004).
- 9 new cases of flu within the school.
- Calculate the incidence during this time period.

**Notes:**
Case study continued.

The school nurse was concerned that 5 out of 100 students had the flu.  She decided to monitor the situation during the month of March (2004) and found that there were 9 new cases of flu within the school.  Calculate the incidence during this time period. 

Incidence will be defined and explained in the following slides.

## Incidence

- The rate at which new disease or health-related condition occurs in a population.

- Incidence = Number of newly affected people

- ___________________________________

- population at risk

- Incidence = 9 new cases of flu

- ___________________

- (780 – 39) population at risk

- Incidence = 12 out of 1,000 students had the flu at the high school

**Notes:**
Incidence looks at the new cases of affected people. To calculate incidence we determine the number of newly affected people (9 in this example) and divide that value by the population at risk.  The population at risk is different than the total population.  If you already have the flu, then you are not at risk of getting the flu.  So the number of those with the flu are subtracted out of the total population to give the population at risk.  For this example we would take the total population 780 minus 39 (those with the flu) to give us a population at risk of 741.  Dividing 9 by 741 gives .012.  Again we multiply by a factor of 10 in this case 1000 to give us the value of 12.  The results are reported as 12 out of 1000 students at the high school had the flu. 

## Case—Study Continued

- Results—12 out of 1,000 students had the flu.
- What is the cause?
- A teacher mentioned seeing students coughing and sneezing around the candy machine.
- The nurse investigated to see if there was any correlation between the candy machine and having the flu.

**Notes:**
Case study continued.

The nurse and the administration at the school became concerned by the amount of school that the students were missing due to the flu.  One teacher mentioned that she had seen students hanging around the candy machine coughing and sneezing.  During the next month, the nurse conducted the following study to determine if exposure to the candy machine was correlated to having the flu.

## Case Study—Continued

- The nurse monitored 200 students over a 1-month period.
- 120 used the candy machine.
- 12 cases of flu occurred in this group.
- 5 cases of flu appeared in the group that _didn’t_* *use the candy machine.
- So was there a risk in using the candy machine?

**Notes:**
Case study continued.

The nurse monitored 200 students over the month.  Of those 200 students, 120 of them used the candy machine.  5 cases of flu appeared in the group that didn’t use the candy machine and 12 in the group that did use the candy machine.  Based on this data, the nurse calculated the risk ratio to determine what the correlation was between the candy machine and the flu.

Risk calculations are given in the following slides

## Prevalence Ratio

- Ratio of prevalence of disease in the exposed group divided by the prevalence of disease in the unexposed group.

**Notes:**
The prevalence ratio compares the incidence (new cases) of disease of those exposed to the possible risk to the prevalence of those people that are not exposed to the risk.  The question in this case study is; Is there an increased risk that I can get the flu from using the candy machine?   A prevalence ratio is a measure of association.  It allows a researchers to determine how strong or weak a relationship is between an exposure (e.g. using the candy machine) and an outcome (e.g. coming down with the flu).  A later slide will discuss how to interpret a prevalence ratio.

## Prevalence Ratio 22 Table

|  | Disease | No Disease | Total |
| --- | --- | --- | --- |
| Exposed | A | B | A+B |
| Unexposed | C | D | C+D |
| Total | A+C | B+D | A+B+C+D |

**Notes:**
To help understand the prevalence ratio calculation we make a 2x2 table (along with the totals).  A represents the number of people that were exposed to and developed the disease.  B represents those that were exposed but did not develop the disease.  C represents the number of people that were not exposed, but still developed the disease.  And D represents the number of people that were not exposed and did not develop the disease.  Totals for each row and column are given to help simplify the calculation.

The prevalence of those exposed in the exposed group is A divided by the total number exposed (A+B).  The prevalence of the unexposed group is C divided by the total number unexposed (C+D).  Therefore, the prevalence ratio is the ratio between these two or; [A/(A+B)] / [C/(C+D)].

## Prevalence Ratio 22 Table

|  | Flu | No Flu | Total |
| --- | --- | --- | --- |
| Exposed to machine | 12 | 108 | 120 |
| Unexposed | 5 | 75 | 80 |
| Total | 17 | 183 | 200 |

**Notes:**
For this example, 12 students were exposed to the candy machine and developed the flu.  108 students were exposed, but did not develop the flu.  5 students were unexposed to the candy machine, but still developed the flu and 75 were unexposed and did not get the flu.

For the prevalence ratio we calculate the ratio between those that were exposed to the machine and developed the flu (12) divided by the total exposed (120) and the number of people unexposed that got the flu (5) divided by the total unexposed population (80).  The prevalence ratio works out to be 1.6.

## Analysis of Prevalence Ratio

- If value >1, then exposure is producing outcome.
- If value =1, no connection between exposure and outcome.
- If value <1, exposure is prohibiting the outcome.
- In the case study, the prevalence ratio was 1.6.  Therefore, students using the candy machine were 1.6 times as likely to get the flu as students not using the candy machine.

**Notes:**
In any ratio, if the value is larger than 1 it means that the numerator is larger than the denominator.  So if the prevalence ratio is larger than 1 it means that the risk of exposure is greater that the risk of not being exposed.  Therefore, using the candy machine is associated with getting the flu.

If the ratio is one, then there is no association between getting the flu and whether you use the candy machine or don’t use the candy machine. 

If the ratio is less than one, then you have a lower chance of getting the flu if you use the candy machine then not using the candy machine.  The use of the candy machine is actually lowering the chance of getting the flu. 

In this case study the prevalence ratio was larger than one.  Therefore exposure to the candy machine was producing an increased risk of getting the flu.

Students using the candy machine were 1.6x as likely to get the flu than students not using the candy machine. 

## Prevention

- The final step of public health is prevention.
- If you were the school nurse, what prevention program would you develop for your school?

**Notes:**
After collecting and analyzing the data, it became evident that exposure to the candy machine increased the risk of getting the flu.  The final step is to share this information with the public and develop a plan to help prevent further spread of the flu.  If you were the school nurse what would you do?

Have the students generate multiple ideas.

Possible solutions:
Have principal disseminate results to faculty
Faculty teach students 
Student leaders become involved
Hand washing campaign
Directions on how to fight spread of germs
Etc.

## Summary

- Pubic health is the science and art of preventing disease, prolonging life, and promoting health through the organized efforts of society.
- There are three parts to public health:
- _Surveillance_, which monitors diseases and health-related conditions.
- _Epidemiology_, which identifies and quantifies trends (prevalence, incidence, and risk ratio).
- _Prevention_, which includes programs that inform and educate the population at risk.

**Notes:**
Public health is organized to help individuals lead a healthier life.  This system is divided into three areas surveillance, epidemiology, and prevention.

## References

- “Public Health Measurement and Research Design” PowerPoint presentation by Alison Nair, CDC Ambassador Program, June 2004.
- Oleckno, W.A. Essential Epidemiology: Principles and Applications. Waveland Press, Inc., Long Grove, Illinois, 2002.