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Lecture 20: Clinical Epidemiology- Measures of Health Status

Terms in this set (47)

Practice Q1
Ans: D

TP rate= Sens = 90/90+10
Distinguishing Proportions, Rates and Ratios

Describe each and give examples
Incidence refers to the # of ..................
New cases only
Incidence proportion is also known as ................... or ...................

Numerator?

Denominator?
Attack rate
-refers to infectious diseases only

Risk

Number of new cases (during specified time interval - ex. last week or last year)

Population at risk
-Refers to those that don't have the disease but are at risk of getting it
Prevalence refers to the proportion of the population at a given time, that ......................

We can calculate the prevalence of what two things? Examples?
Has the factor of interest

1. Of an exposure -> refers to a risk factor
-Ex. what proportion of this class has a BMI > 25

2. Of an outcome or condition
-Ex. what proportion of this class has anxiety
Prevalence of a given disease is used as a measure of .......................
The burden of disease for society
-Of all diseases, which one is the most prevalent
What is the Formula to calculate Prevalence?
Prevalence = Total Number of Cases/Total population

Numerator -> all those with the attribute at a particular time -> everyone who has the disease

Denominator -> population having the attribute during that same time period (often, size of the population) -> everyone in the population
Point prevalence refers to .......................
Total # of people with the disease/population

At any point in time

Ex. Prevalence of diabetes in Grenada
Period prevalence refers to .......................
Prevalence during a specified period or span of time

Ex. Prevalence of diabetes in Grenada in 2021
Point prevalence in a population at one time is calculated as ................
# of Cases/#of Persons

-Prevalence of these 10 people?
-We have 1 case
-Thus prevalence = 1/10 = 0.1 = 10% of population has disease
Fixed populations refer to ................ populations. Membership is based on ............... and is ....................

Dynamic populations refer to ......................... populations. Membership is based on a ................. and is .................
Closed
-No one is leaving and no one is entering

An event
-Ex. Japanese atomic bomb survivors

Permanent

Open
-People are leaving and entering

Condition
-Ex. residents of a city, hospital patients

Transitory
Design of a prevalence study

Ex. What is the prevalence of hypertension in Grenada?

How do you carry out this study?
We take a sample of the population of Grenada, that is representative
-Meaning that its representative of the entire population, so you have to go to all areas of Grenada to get this sample

-Eg: 500 in sample - 20 people has disease so prevalence is 20/500 = 0.04 = 4%
What is the Formula to calculate Incidence?

Also known as .....................
Incidence = # of new cases/population at risk

Population at risk refers to those WITHOUT the disease

Incidence proportion
What is the Formula to calculate Incidence rate?

Also known as .....................
Incidence rate = # of new cases/person-time at risk

Incidence density

Person-time can be days, months, years etc.
Incidence Rate
-Study from Jan 1 - Dec 31

-At start of study ie Jan 1 --> look at those who don't have the disease that we are measuring

-Those who develop the disease will be the new incident cases

-Incidence = new cases/baseline population at risk (jan 1)

-And since we are following up after a year we will consider time

-Eg: we followed 500 people for a year - person time at risk - 500 x 1 --> Incidence= 10 (new cases)/500 (pop) x 1 (year)= 0.02 cases per person year à x100 --> 2 cases per 100 person yearsà we followed 100 people for a year and got 2 cases
What is the formula for prevalence?

What is the formula for incidence?
prevALence = ALL cases/Population

iNcidence = New cases/Population at risk
Cumulative incidence refers to ............................
How many cases accumulated during a period of time

Ex. The US has the highest number of cases since the COVID-19 pandemic started

It is still calculated as:
New cases/Population at risk
Characteristics of Incidence and Prevalence
Populations are considered .................. because they are ...................
Dynamic

Constantly changing

-Population is dynamic ie it is constantly changing
-People leaving pop: die, emigrate
-People entering pop: birth, immigration
Population Incidence Changing Factors

Based on the formula for prevalence, how can we INCREASE it?

Based on the formula for prevalence, how can we DECREASE it?
By increasing the # of cases or by decreasing the population
-Meaning that if people die or migrate from the population, your prevalence will increase

By decreasing the # of cases or by increasing the population
-Meaning people recovered or people entering the population
Be able to Tell if the Incidence and Prevalence increase/decrease or stay the same

Is increasing prevalence necessarily a bad thing?
Know!

No
-It means the treatments are not effective because it doesn't cure them, but you are keeping patients alive a lot longer
-Ex. Patients with alzheimers disease and diabetes
What happens to the incidence and prevalence if:

New EFFECTIVE treatment is initiated?
Incidence = No change

Prevalence = Decrease
-Because this new EFFECTIVE treatment is CURING the patients, meaning that the # of cases is going to decrease

Remember the formula:
Prevalence = # of all cases/population
What happens to the incidence and prevalence if:

New EFFECTIVE vaccine gains widespread use?
Incidence = Decrease

Prevalence = Decrease
What happens to the incidence and prevalence if:

Additional federal research dollars are targeted to a specific condition?
Incidence = No change

Prevalence = No change
What happens to the incidence and prevalence if:

Behavioral risk factors are reduced in the population at large?
Incidence = Decrease

Prevalence = Decrease
What happens to the incidence and prevalence if:

Contacts between infected persons and non-infected persons are reduced for:

-Airborne infectious disease?

-For noninfectious disease?
Incidence = Decrease
Prevalence = Decrease

Incidence = No change
Prevalence = No change
What happens to the incidence and prevalence if:

Recovery from the disease is more rapid than it was 1 year ago?
Incidence = No change

Prevalence = Decrease
What happens to the incidence and prevalence if:

Long-term survival rates for the disease are increasing?
Incidence = No change

Prevalence = Increase
Incidence density/rate measures the number of .................... in a ..................... population, with people ............ and ............

It provides the ........................ of disease in ........... and ............, using a ....................... for the outcome of the event
New cases

Dynamic

Entering and leaving

"Density" of new cases

Time and place

Person-time at risk
Incidence Rate formula
-We are following 8 people who didn't have the disease
-Horizontal line --> period at which they are being followed
-X= got cases=3, people being followed- 8 thus:

-Incidence = 3/8
-Incidence rate= 3/73 months ie 3 cases per 73 months follow up or 3 cases per 6.1 years (73/12) of follow up
How can we calculate the prevalence using incidence?
Prevalence = Incidence x Average Duration of disease

Rearrange the equation to find duration:

Duration = Prevalence/Incidence
What is the Formula to calculate Attack Rate?
Attack Rate (or incidence) = # of episodes during a specified period/Pop. at risk/period x 10^n

This refers to infectious diseases!!!!!!

- n= 1-10 people, n=10 - 100 people etc
Attack Rate
- Looks like flu since incidence is high during winter months
What formula can be used to calculate the crude birth rate?

It is conventionally expressed as per ....................
Crude birth rate = Live births/Population
-It is the ratio of live-births to the population in the reference period

Thousand population
Practice Q2
Ans: D

- 500 total
- 50 cases in beginning of 2021
- new cases: 24 other women+ women recently diagnosed= 25 new cases
- total cases= 50+25=75
- 4 died so new pop is 4996 and total cases is 75-4=71
- thus 71/4996
What formula can we use to calculate RISK?
Risk = chances of SOMETHING happening/chances of ALL things happening

Think of this as:
the risk of something happening / the risk of something happening + the risk of something not happening

A proportion

-risk --> disease/all population

Example: Risk of getting disease/risk of getting disease + risk of NOT getting disease
What formula can we use to calculate ODDS?
Odds = chances of SOMETHING happening/chances of it NOT happening

-Odds --> disease/no disease

This is a ratio
Probability vs Odds
-Risk is a proportion
-Odds is a ratio
Confidence Interval
--Narrow ie 68% --> more precise but more error
-Wider ie 99% --> less precise but less error
The Confidence Interval (CI) is .................. related to Precision

The wider the CI, the ..................... precise the estimate and The narrower the CI, the .................... precise the estimate
Inversely

Less
-Less precise but more accurate (less chance of making an error)

More
-More precise but less accurate
99% CI is ................. accurate but ................. precise

95% CI is ................. accurate but ................. precise
More
-Only 1% chance of error

Less

Less
-5% chance of error

More
How does an increase in sample size affect the precision?
Increase sample size = Increase precision
Risk factor refers to a ................... associated with ......................
Patient characteristic

The risk of contracting a specific disease
Prognostic factor refers to a ................... that identifies ...................... having ......................
Patient characteristic

Subgroups of untreated patients

Different outcomes
Factor predictive of treatment effect refers to a .................. that identifies ..................... having ............................
Patient characteristic

Subgroups of treated patients

Different outcomes
-As a consequence of treatment
Practice Q3
Ans: C

Attack rate= number of new cases/people at risk

11 mill people at risk, 540 got it thus 540/11 mill
How can we calculate case fatality rate?
# of people that died / # of people sick

ex. 17/540
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