lecture 11 script cont' Frequency + Risk

8/3/2019 lecture 11 script cont' Frequency + Risk

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Frequency (2) and RiskResearch Methods

Lecture # 11

Prevalence

Today we will continue the topic Prevalence, but before

continuing this topic just commenting on the issue that we raise

last time, about the denominator in the calculation of prevalence..

To keep things more clear

Remember : Prevalence is the proportion of a group of people with a clinical

condition at a given point in time.

We discussed something important last time, we said if you wont

to calculate prevalence at the beginning of 1992 (this example

about the incidence of cancer among population studied between

1991 and 1993, slide#8).

- So we dont have any problem in calculate prevalence here

at the beginning of the study, Because simply here we have

100 subjects, those 100 subjects at the beginning 4 of them

had cancer so

Prevalence = 4/100 (its simple)

- But here if we want to calculate the prevalence at this slide

which is the beginning of 1992, we have to exclude those

who died, For example this case develop cancer and

continuo maybe until two months and this case die !! So

thats why because this case die, we cant include this case

in the denominator ().

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This depends on what we mean by the stopping of the cancer, I

mean here if we consider that this case died then of course it

shouldnt be incorporator in the denominator. But if we

consider that the stopping of the cancer means that the patient

was cured. Can we incorporate this patient again in the study? Ordo you think its fair to include in the study people who develop

cancer and they are cured? I think its not fair or its betternot

to includethem but actually its not a concept!!

So ..

If the case die 100% it will not included in the study, so if we

mean by stopping the cancer the death for sure we will

excluded from the denominator.

If we consider that the stopping of the cancer means that

the patient was cured .. here we have a question mark !!

( If I am a researcher I dont include people with previous

experience of cancer )

But for the EXAM, if we tell you that the case dies dont include

it

(and there will be no question means that the case was cured)

- Or sometime the case drop from the study (it wont

contributes) also in this case you have to exclude those

cases who cant be included in your study.

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Incidence

In Incidence study we usually study incidence by cohort study,this means we identify a group of people that have something in

common and then we follow this group up for a period of time. In

prevalence we start the study by cross sectional study, thats

mean we come to group and examinants at once (single point of

time) and then we calculate the cases that we want.

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Remember: Incidence is the proportion of a group of people initially free of

the condition that develops the condition over agiven period of time

Thats why we cohort study all people should be the same, but inprevalence study not all the people should be necessarily the

same. For example if I want to study cancer in my students (

) in prevalence study at this single point of time I just

calculate those with cancer so this is the nominator () then

the denominator is the total number of my students .

But if I want to study the incidence of cancer in this group of

people, I have to determine the beginning of the study which

today for example and the study have to end after 5 years.

During this period of time I count new cases of cancer, this mean

at the beginning of study anybody with cancer should be

excluded. Only we should include non-cases, because its not

logical at the beginning of my study I have people with cancer. I

want to study during period of time to know who develop a

cancer, so thats why in my study with incidence I need a group of

people with very much similar characteristics. But in prevalence

people should not be the same.

And I want to clarify that if its very impossible for your

subjects to develop the disease or to develop the

outcome, dont include in your study.

I gave you an example lets assume we want to study cervical

cancer and we have women who underwent hysterectomy its notlogical to include those women in my study because they have

removed their uterus, thats why we should have a base line

examination, we have to exclude cases that are impossible to

develop the outcome, another example How can you study

caries on a patient that is edentulous?

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examination is the examination itselfwe examine them once

those with problems have to be excluded.

In the doctor research he excluded those with high BMI (Body

Mass Index) too fat or too thin because obesity have someeffect on tooth eruption and people with bone disease have to be

excluded, also people with orthodontic treatment because all of

these affect timing of tooth emergence, however I may include

them on a study on their own like the effect of body mass index

on timing of tooth eruption.

Identifying cases free from the condition, I have to identify people

who are free from the condition if we have a case with a condition

we have to exclude it, but in prevalence I have to include it,following up to identify new cases with the condition.

Prevalence and incidence Vs. duration

Increasing duration of disease increases the chance that the

patient will be included in a prevalence study, if the duration of

the disease is long for example some cancers lead to death andothers not, for example hepatic pancreatic cancer will lead to

death in short period of time, while patients with Hodgkins

lymphoma are curable and live for long time. Now if he have long

survival rate he has a great percentage in being included in the

study because anytime you want to do that study you will find

that patient is alive or still with the condition, diseases of short

duration are more likely to be missed by a future prevalence

study.Crohns disease is a chronic disease so any time you want to

make a study you will find that case ready but congestive heart

disease (CHD) has short duration due to congestive heart failure.

Prevalence is better with diseases of long duration

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Incidence is better with diseases of short duration

Caries which is a short duration disease within 1-2 years the

study will end but Crohns needs about 20-40 years.

Case fatality rate

Other frequency rate that we have in addition to incidence and

prevalence we have what we call "case fatality rate" which is

mean the proportion of people with condition who die of it.

For example, I follow up 100 cases of women having breast

cancer for 5 years and 5 of them died because of breast cancerthan 5% will be case fatality rate.

Follow up duration should be long enough for potential death to

have occurred. Seems not logical to study breast cancer for 3

months only! May be none of those women die during this short

period!

At least I must study them for at least 5 years to count these

cases that have been died due to disease.

Case cure rate

Let us assume that we follow 100 women with breast cancer for 5

years, I count the number of cases who are totally cured from

breast cancer so this is case cure rate.

Case complication rate

Those cases who developed complications!

Infant mortality rate

Infant that dies after birth, there is a common syndrome called

"infant sudden death syndrome "is the sudden and unexplained

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death of an infant who is younger than 1 year old. It's a

frightening prospect because it can strike without warning,

usually in seemingly healthy babies. Most SIDS deaths are

associated with sleep (hence the common reference to "crib

death") and infants who die of SIDS show no signs of suffering.

I want to study infant mortality rate, I study children from birth to

6 months then I count the cases that have mortality.

Prenatal mortality

The number of cases that havent born yet and died .

All of these frequencies are incidence rate, not prevalence rate

because I follow them for a specific duration of time.

Slide 16 & 17 NOT included

Interpreting temporal sequenceIn incidence study the population should be free of the disease

and follow up to measure the development of that disease. BUT in

prevalence study, population of existing cases and also non cases

of the disease these are studied at once (cross section the time)

to measure cases with no reference to when they have been

occurred.

The problem in the prevalence is that you study the case and youhave no idea when the disease has been occurred. Let us assume

that we want to study caries in students, I account the students

with caries and I found that they were 5% BUT I didnt know when

the caries started to develop.

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In incidence I have the case and he develop the disease under the

study, I can follow up the case and determine exactly when the

case start to develop the disease < increase, decrease,> so the

incidence give u a better idea about the duration that the disease

has taken, also what happens during the development of diseasein terms of .

Old and new cases

Prevalence studies include old and new cases of disease. Cases included are only the cases available at the time of

examination. The cases in the numerator are different between

prevalence and incidence studies.

I have the old and new cases, if I want to study incidence, I have

to exclude those old cases BUT in the prevalence study the old

cases that already have the outcome will be included.

In slide # 20 we have a longitudinal study, if you want to

study the incidence some of cases may enter the study after a

period of time (after a month for example), another is cured,

another left the study, so in the incidence I have a possibility to

some people to get in or get out BUT in the prevalence you

examine the patients once so those will be present in a point of

time exactly.

Determines of disease distribution:

Time Place

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Person

Distribution by the time:

Epidemic

When the concentration of new cases in a period oftime substantially exceeds what is normally expectedbased on recent experience / research .(locally)

Let us assume the level of the flu in Jordanian people at

any point of time is 5%. If the flu increased suddenly to

25% or 30% at once in this case we say flu is epidemic.

Note that substantially exceeds not from 5% to 10%. Itis from 5% to 30% for example.

Pandemic

Disease widespread across a large region such as awhole continent or globally (H1N1 influenza)

Now what are prevalence studies good for?

Cross-sectional studies are the basis of diagnostic

testing.

Planning health services related to common diseases

such as diabetes and hypertension in Jordan.

Let us assume that 30% of Jordanians have diabetes and 40% ofthe Jordanians have hypertension THEN WHAT!! Ministry of health

will improve the services to reduce these percentages.

Slide 24 & 25 NOT included

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RiskRisk is the probability of some untoward effect.

Definition

The probability that people who are exposed to certain risk

factors will subsequently develop the disease more often than

similar unexposed people.

For example you want to study the effect of smoking on lungcancer. Here the smoking is the risk factor. Why? Because people

who are exposed to this risk factor (smoker) they have higher

possibility to have cancer than other.

- Factors associated with an increased risk of becoming diseased

these are called Risk factor.

This lecture describes how estimates of risk are obtained by

observing the relationship between exposure to possible risk andthe subsequent development of the disease:

Sometimes we look forwards and sometimes we look

backwards.

Sometime in the study I begin with group of people then the

study over period of time in this case I go forward called

prospectivestudy ( )

Sometime in my study I can go backward: I go to the record or

files of the patient and this called retrospective study (

) for example I go back to the files of patients in the years

from 2000 to 2010 and I see who was a smoker and developed

lung cancer.

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But the problem in retrospective studies are not very efficient in

people with bad filing system. Like here in Jordan unfortunately

we dont have good filing system a lot of time the files aremissing or the doctor doesnt describe the case very well in the

file so in developing countries we cant rely on the

retrospective study but in developed countries we can efficiently

depend on retrospective study.

Risk factors can be:

Physical environment factors: Toxin, infectious agents,gas, pollutants.

Social environment factors: Emotional illness, stress,

loss of family members, culture.

Behavioral factors: Smoking, driving without seat belts,

inactivity.

Inherited factors: Diabetes, cholesterol, triglyceride.

Exposure to risk factors

-The exposed person:

Has come in contact with risk factor

Or has manifested the factor in question

You must study him Before becoming ill

-Duration of exposure:

-The risk factor affect at a single point in time, example

nuclear bomb in Hiroshima affect people at that time.

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- Or over a long period of time likes the effect of smoking.

Amount of exposure

We ask relevant questions:

Have you ever been exposed?

What is the current dose?

Largest dose taken.

Total cumulative dose.

Years of exposure.

Years since first exposure.

These questions we ask them about any risk factor. In Dentistry

the risk factor that we most commonly ask about is smoking. We

ask the patient when you started smoking , how many times do

you smoke in a day? And so on ..

The Doctor skipped a group of slides .. and then :

Cohort study

The other synonyms of the cohort study are:Incidence studies,longitudinal studies, Prospective studies.

Cohort:People assembled have not experienced the outcome(didnt have the disease before) but have equal susceptibility to

develop the outcome. Thats why you cant include women with

hysterectomy (removal of the uterus) in study the incidence of

the cervical cancer.

So all the assembled people must have equal susceptibility to

develop the disease but no one have the disease in the beginning

of the study.

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Big thanks go to : Ruba Ghanim and Noor Bdeir

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lecture 11 script cont' Frequency + Risk