Epi Methods M1

What is the goal of epidemiology?

To measure how often disease happens and how exposures affect disease, using results that are as accurate and reliable as possible. 🎯🦠

What does valid mean in epidemiology?

A measure is valid if it is both accurate and precise. ✅🎯

What does accurate mean?

It means your result is close to the true answer. 🎯

What does precise mean?

It means you get similar results again and again, with little random variation. 🔁📏

What is risk?

Risk is the chance that a person will develop a disease over a certain period of time. ⏳🦠

What is another name for risk?

Incidence proportion. 🔄📘

What counts in the numerator of risk?

Only new cases that happen during the time period. ➕🦠

What counts in the denominator of risk?

People who were at risk of getting the disease at the start of the time period. 👥

Why is risk a proportion?

Because the people who become cases are part of the group that was at risk to begin with. 🧩📊

Why must the time period be stated when talking about risk?

Because risk changes depending on how long people are followed. ⏰📅

What is a competing risk?

It is when someone dies from another cause before they could develop the disease being studied. ⚰️➡️❓

Why are competing risks a problem for calculating risk?

Because we no longer know whether that person would have developed the disease later. 🤷‍♀️📉

What is loss to follow-up?

It means researchers lose contact with someone before the study ends. 🚶📭

Why is loss to follow-up a problem?

Because we do not know what happened to those people after they left the study. ❓👤

What is a dynamic population?

A population where people can enter and leave over time. 🔄👥

Why is a dynamic population hard for risk calculation?

Because risk assumes a fixed group at the start, but dynamic populations keep changing. 📈📉

When is risk easiest to calculate?

When the population is closed and there are little or no competing risks or loss to follow-up. 🔒✅

What can we use if risk is hard to estimate?

Incidence rate. ⏱️📊

What is incidence rate?

It tells us how quickly new cases happen in a population. 🚀🦠

What is in the numerator of incidence rate?

New cases of disease. ➕🦠

What is in the denominator of incidence rate?

Total person-time at risk. 👥⏳

What is person-time?

It is the total amount of time people are actually observed and at risk. ⌛👥

Why is person-time useful?

Because it gives each person credit for the actual amount of time they were followed. 🎓⏱️

What units are used for incidence rate?

Usually cases per person-time, like cases per 1,000 person-years. 📊📆

What is the main advantage of incidence rate?

It works better when people are followed for different amounts of time. ✅⏳

What is the main problem with incidence rate?

It is harder for people to understand than risk. 😵📉

What is waiting time?

The average time until the next event happens. ⌛➡️🦠

How is waiting time related to incidence rate?

It is the inverse of the incidence rate. 🔄➗

Why do epidemiologists sometimes convert rates into risks?

Because risk is easier to understand and explain. 🗣️📘

What formula connects incidence rate and risk?

The exponential formula. 📐✨

When does risk approximately equal incidence rate times time?

When the disease is rare or the time period is short. 🤏⏳

Why should you be careful with the shortcut formula?

Because it becomes less accurate when risk is not small. ⚠️📉

What is prevalence?

The proportion of people who have a disease at a specific point in time. 📸🦠

What counts in the numerator of prevalence?

All existing cases at that time point. 🧍‍♀️🧍‍♂️🦠

What counts in the denominator of prevalence?

The total population at that time point. 👥

How is prevalence different from risk?

Prevalence is about who has disease now; risk is about who develops disease over time. 📸⏳

What two things affect prevalence?

How often disease happens and how long it lasts. 🦠➕📆

Why can a disease have low incidence but high prevalence?

Because people may live with it for a long time. 🐢📈

Why is prevalence not ideal for studying causes of disease?

Because it mixes together new disease and disease duration. 🔀🧠

What are measures of effect?

Ways to compare disease frequency between exposed and unexposed groups. ⚖️📊

What are the two main types of measures of effect?

Difference measures and ratio measures. ➖➗

What does a difference measure tell you?

How many more or fewer cases happen in one group compared with another. ➕➖👥

What does a ratio measure tell you?

How many times higher or lower the disease frequency is in one group compared with another. ✖️📊

What is a risk difference?

The risk in the exposed group minus the risk in the unexposed group. ➖🦠

What does risk difference mean in plain language?

It tells you the extra number of cases linked to the exposure. ➕📈

What is an incidence rate difference?

The incidence rate in the exposed group minus the incidence rate in the unexposed group. ⏱️➖

What is a risk ratio?

The risk in the exposed group divided by the risk in the unexposed group. ➗📊

What does a risk ratio of 2 mean?

The exposed group has twice the risk of disease as the unexposed group. 2️⃣⬆️

What is an incidence rate ratio?

The incidence rate in the exposed group divided by the incidence rate in the unexposed group. ⏱️➗

When are risk ratios and rate ratios close to each other?

When the disease is rare or the follow-up time is short. 🤏📅

What happens to the risk ratio over long follow-up?

It moves closer to 1 and may underestimate the rate ratio. ⏳➡️1️⃣

Why are difference measures important?

Because they show the actual number of extra cases and help with public health planning. 🏥📉

Why are ratio measures important?

Because they show the strength of the relationship between exposure and disease. 💪🔗

What is attributable risk?

The extra disease risk among exposed people that is due to the exposure. 👥➡️🦠

What is another way to think about attributable risk?

How much the exposed group's risk would drop if the exposure were removed. 🚫📉

What is attributable fraction?

The proportion of disease among the exposed group that is caused by the exposure. 🧩📊

What does attributable fraction tell you in plain language?

Out of all the cases in exposed people, what percent are due to the exposure. 💯👥

What is population attributable risk?

The amount of disease in the whole population that is due to the exposure. 🌍📉

What is population attributable fraction?

The percentage of all cases in the whole population that are due to the exposure. 🌎💯

Why are population attributable measures useful?

They help show how much disease could be prevented in the whole population. 🛡️🌍

Why is attributable fraction always bigger than population attributable fraction?

Because removing the exposure helps the exposed group more than it helps the whole population. 👥⬆️🌍

What is an odds ratio?

A measure that compares the odds of disease between exposed and unexposed groups. 🎲➗

When is the odds ratio especially used?

In case-control studies. 📘🔍

Why can’t case-control studies directly calculate risk?

Because the number of cases and controls is chosen by the study design, not by the true disease frequency in the population. ⚙️🚫

What measure of effect is usually valid in a case-control study?

The odds ratio. ✅📊

What impact measures can still be estimated in case-control studies?

Attributable fraction and population attributable fraction. 📈🌍

What is the big-picture difference between disease occurrence and disease effect?

Disease occurrence asks how much disease there is; disease effect asks how exposure changes disease. 🦠⚖️

What is the big-picture difference between effect and impact?

Effect tells you how strongly exposure is linked to disease; impact tells you how much disease could be prevented. 🔗🛡