slide 5 frequency

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FREQUENCY

Research Methods

Dent 313



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Frequency

Questions that clinicians need to answer in the care of their patients take the form of probability

Frequencies obtained from clinical research are the

basis for probability estimates in patient care Probability can be expressed in words instead of

numbers

Avoiding precision when uncertain

Subjectivity (usually, rarely)



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Frequency

Measuring the frequency of an event is expressed by afraction

Numerator = No. of patients experiencing the outcome(Cases)

Denominator = Total number of patients (number of people in whom the outcome could have occurred)

Frequency Denominator

Numerator=



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Basic measures of frequency

Prevalence

is the proportion of a group of people with aclinical condition at a given point in time

Incidence is the proportion of a group of people initially free

of the condition that develops the condition over a given period of time



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Prevalence

Measurement is at a single point of time

Point prevalence

Single point of time for each patient

Not necessarily a single point of time for all patients

Period prevalence

Describing cases present at a specified “short” period of time

Does not discriminate old from new cases

Conducted by surveying a defined group of peoplewith or without a condition of interest

Those who are having the condition at the time of examination are counted (numerator) against aspecified population (denominator)



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Incidence

Following up new cases of disease occurring

(numerator) in a specified population initially

free of disease

Measurement is periodically over an interval

of time

Should define the time interval first



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Denominator (specified population)

Specified population is the population at risk

of developing the outcome (denominator)

E.g., cervical cancer (can’t include women with

hysterectomy?)

Related to the research question

E.g., prevalence of Carabelli’s cusp in the Jordan

Should include random sample representative of all the

Jordanian population



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1991 1992 1993

Onset of a cancer

Duration beingin the study

Measurement of incidence and prevalence of cancer amonga population from 1991-1993 (n=100)



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1991 prevalence = 4/1001992 prevalence = 5/1001993 prevalence = 7/100

1991 1992 1993

Onset of a cancer




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1991 incidence = 5/961992 incidence = 6/9191-93 incidence = 11/96

1991 1992 1993

Onset of a cancer




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Incidence Prevalence

NumeratorNew cases during aperiod of time

All cases countedon a singleexamination

Denominator

All susceptible

persons at thebeginning of theperiod

All people

examined includingcases and non-cases

Time Duration of the period Single point (cross-section of time)

How measuredCohort study (cohort: a

group having something in

common when assembled)

Cross sectionalstudy



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Measuring prevalence

Prevalence study or Cross-sectional study

The most common studies in the literature

Studies need less time

Studies need less resources

Studies need less statistical analyses

Studies are less complicated



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Measuring Incidence

Incidence studies or cohort or longitudinal

studies

Baseline examination (as in prevalence study)

Identifying cases free from the condition

Following up to identify new cases with the

condition



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Prevalence & incidence vs. duration

Increasing duration of disease increases the chance

that the patient will be included in a prevalence study

Diseases of brief duration is more likely to be missed

by a future prevalence study E.g., CHD vs. Crohn’s disease

Prevalence is better with diseases of long duration

Incidence is better with diseases of short duration



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Other frequency rates

Case fatality rate

Proportion with a condition who dies of it

Follow up duration should be long enough for

potential deaths to have occurred

Case cure rate

Case complication rate

Infant mortality rate

Perinatal mortality rate



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Bias in prevalence studies

Temporal sequence

Old and new cases



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Interpreting temporal sequence

In Prevalence studies

Disease and possible causal factor are measuredsimultaneously

It is not clear which one come first The time dimension is lost

In Incidence studies

There is a build in the sequence of causal factorsand diseases



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Measuredevelopment ofnew cases ofdisease over time

Measure caseswith no referenceto when they haveoccured

Population

free ofdisease

Population ofexistingcases/ non-cases of

disease

Incidence study

Prevalence study

Interpreting temporal sequence



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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



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EarlyDeath

Cures Leavestudy

Enter study

Incident cases(All new cases

arising in adefined

population)

Prevalentcases

(present atpoint in

time)

Old and new cases



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Determinants of disease distribution

Time

Place

Person



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Distribution by time

Epidemic

When the concentration of new cases in a period of time substantially exceeds what is normally

expected based on recent experience / research Pandemic

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



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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 inJordan



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When aren’t prevalence studies indicated?

Weak evidence of cause and effect

Causal question are inherently about new events arisingover time (they are about incidence)

Weak evidence of which came first (cause or effect)because they are measured simultaneously

Prevalence may be the result of

Incidence of disease

Duration of disease



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Uses of prevalence and incidence

studies

Predicting the future

Used to predict the probability of similar peoplewho will develop the disease in the future

The probability that the patient has thecondition

Prevalence is determinant of predictive value

Making comparisons It is a tool to make a clinical decision in

comparison between exposed and non-exposedpeople

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