Measuring Disease Frequency Dr. Akhilesh Bhargava MD, DHA, PGDHRM Prof. Community Medicine & Director-SIHFW, Jaipur

Measuring Diseases/ Events:

Akhilesh Bhargava 2 Measuring Diseases/ Events Epi . Objective: Distribution Comparison with reference to population size and, Time For Prioritization Trend study Resource allocation

Slide 3:

Akhilesh Bhargava 3 The fundamental task in epidemiologic research is to quantify the occurrence of illness The goal is to evaluate causation of illness by relating disease occurrence to characteristics of people and their environment Rates, Ratios, and Standardized Rates are primary tools for quantifying occurrence of illness

Counting Tools:

Akhilesh Bhargava 4 Counting Tools Rate Ratio Proportion

Rate?:

Akhilesh Bhargava 5 Rate? “a measure of speed with which events are occurring in a population in a specified time period.” A numerator A denominator that “appropriately” relates the numerator to population at risk A “unit” such as per 1000, per 100,000 or per million

Why a rate?:

Akhilesh Bhargava 6 Why a rate? To ensure comparing apples with apples

Ratio: “a fraction (a/b) of two mutually exclusive groups with elements “a” & “b”. :

Akhilesh Bhargava 7 Ratio: “a fraction (a/b) of two mutually exclusive groups with elements “a” & “b”.

Proportion: “a fraction (a /a + b) of two mutually exclusive groups with elements “a”& “b” :

Akhilesh Bhargava 8 Proportion: “a fraction (a /a + b) of two mutually exclusive groups with elements “a”& “b” +

Slide 9:

Akhilesh Bhargava 9 Index Numerator Denominator Proportion People with disease All people with & without disease Ratio People with disease People without disease Rate People with disease in a given period All people with & without disease

Counting diseases:

Akhilesh Bhargava 10 Counting diseases Mortality Tools Crude Mortality Case fatality Proportional Mortality Standardized Mortality Age specific Mortality Morbidity Tools Prevalence Incidence

Morbidity Measuring: Prevalence-:

Akhilesh Bhargava 11 Morbidity Measuring: Prevalence- Prevalence - Total no. of cases ( new + old) ----------------- x 100 Total population over specified period Point prevalence Period prevalence Prevalence rate- a Ratio, reflects status

Population at Risk- in a study of Cancer cervix:

Akhilesh Bhargava 12 Population at Risk- in a study of Cancer cervix All Men All Women 0-25 Yrs 25-69 Yrs. 70+ 25-69 Yrs. All Women Pop. At Risk Total Population Only Pop. At risk should go into denominator of Prevalence rate but at times it is the total population that is considered

Prevalence::

Akhilesh Bhargava 13 Prevalence: Prevalence can be expressed either as a proportion or as a rate Expressed as a proportion, prevalence is a number between 0 and 1 As a rate, prevalence can be expressed as per 1000, per 100,000, or per whatever

Prevalence-Types:

Akhilesh Bhargava 14 Prevalence-Types Point Period So far as prevalence is concerned it generally refers to point prevalence. However when the period of observation is large it is referred as period prevalence where the numerator will have all existing cases plus all new cases occurring during period of observation an denominator will be mid year population

Prevalence: Example:

Akhilesh Bhargava 15 Prevalence: Example In a sample of 1,038 women (70-74 years), 70 were found to have rheumatoid arthritis. The prevalence of arthritis is: 70 P= ------------------ =0.07 per women ( 70-74) 1,038 Or P= 70 per thousand women age 70-74 Or P= 7 percent for women age 70-74 Or……….

Prevalence::

Akhilesh Bhargava 16 Prevalence: Choice of scale of rate usually depends on the ubiquity of the disease. Thus, more common disease prevalence may be presented as percentage Rare disease prevalence may be presented as per 100,000 or per million

Slide 17:

Akhilesh Bhargava 17 In 2004 there were 1076 cases of Tuberculosis in District X among 50000 men in age group of 40-44 years. The Prevalence rate will be: 1076 p = ________ =0.0215 per year 50000 = 21.5 per thousand per year = 215 per 10 thousand per year = 2150 per million per year

Change in Prevalence reflects:

Akhilesh Bhargava 18 Change in Prevalence reflects Change in incidence or duration of disease Introduction or impact of an intervention Selective attrition Change in disease definition or classification Significant migration

Prevalence has its use in – :

Akhilesh Bhargava 19 Prevalence has its use in – Determination of the sickness load Planning of health services in relation to Infrastructure Manpower Facilities, and Finances In making community diagnosis

Incidence Rate:

Akhilesh Bhargava 20 Incidence Rate number of new cases occurring during a period of time I = x 100 “total person time” at risk What is “person time”: The duration of time a person is at risk Usually expressed as person years but can be expressed as anything, e.g., person months, person weeks, etc.

Incidence Rate-Types:

Akhilesh Bhargava 21 Incidence Rate-Types Cumulative Incidence (CI): New cases --------------------------------------- X 100 Population at risk (PAR) during a specified period Incidence Density (ID)/ Incidence rate New cases ------------------------------------------- X 100 Total Person time of observation in PAR, over a specified period

“Total Person Time”:

Akhilesh Bhargava 22 “Total Person Time” Sum of person time of all individuals who were at risk and were available for observation. Equivalence of “total person time” 50,000 person years = 5,000 persons observed for 10 years = 1,000 persons observed for 50 years = 10,000 persons observed for 5 years

Slide 23:

Akhilesh Bhargava 23 Say In 2009 there were 1139 cases of Measles in City “X” (Pop.-2500000, children- 15%) among children 0-5 years. The number of person years was 375000. The incidence rate will be: 1139 I = ------------- =0.00317 per person per year, 375000 or = 3.17466 per thousand per year, or = 31.7066 per 10 thousand per year, or = 3170.666 per million per year To be more accurate, we must add another qualifier, namely, “for children 0-5 years of age”

Slide 24:

Akhilesh Bhargava 24 This would mean exclusion of people currently having disease people who had had the disease people who are protected on account of-immunization, habits and earlier intervention; from the population at risk

Incidence Rate: Expressed as-:

Akhilesh Bhargava 25 Incidence Rate: Expressed as- Morbidity rate- New cases\total population at risk Mortality rate- No. Of deaths due to a disease\total population Case fatality rate- No. Of deaths due to a disease\total no. Of cases of that disease Attack rate- No. Of cases of a disease, not persons / total population at risk for a very short period

Change in Incidence reflects:

Akhilesh Bhargava 26 Change in Incidence reflects Introduction of a new risk factor Changes in habits Change in virulence Change in intervention strategy Selective migration

Incidence useful in :

Akhilesh Bhargava 27 Incidence useful in surveillance understanding etiology & pathogenesis & planning of new services

Prevalence V/S Incidence:

Akhilesh Bhargava 28 Prevalence V/S Incidence Prevalence: A “snapshot” of disease at a point in time in a population Incidence: A description of how new cases of disease are occurring. “force of morbidity” “rate of flow” of cases from non disease to disease state

Prevalence (P) and Incidence (I):

Akhilesh Bhargava 29 Prevalence (P) and Incidence (I) P= I x d d=duration If the disease is stable, that is, if the incidence and duration remains constant over time.

Slide 30:

Akhilesh Bhargava 30 Died Migrated out Migrated out Died Pop. On Jan 1, 2004= 25000 July 1, Mid Year Dec. 31 =100 Cases

Slide 31:

Akhilesh Bhargava 31 Point prevalen ce on Jan.1, 2004 = 400 / 25000 x 100 = 1.6 % Point prevalence on July, 1, 2004 = 600/ 24800 (i.e. 25000-200) x 100 = 2.41 % Point prevalence on December 31, 2004 = 400/ 24600 (i.e25000-400) x 100 = 1.62 Period prevalence in one year = 400+1100 / 25000-200 x 100 = 6.51% Cumulative Incidence for the year (Jan.1, 2004-December 31, 2003) =1100/ 25000-400x 100= 4.47

Slide 32:

Akhilesh Bhargava 32 Date of Onset of disease Date of Termination or death R Case no . 1 2 3 4 5 R Date of recurrence R = July 30, 2003 June30, 2004 N= 300 Point prevalence on July 30, 2003= 4 cases( 1, 2, 3, 6)/ 300 Incidence rate on July 30, 2003= 2 cases (4, 5) /296 Period prevalence between July 30, 2003 to June 30, 2004= 6 /300 6

Prevalence Vs. Incidence:

Akhilesh Bhargava 33 Prevalence Vs. Incidence Prevalence: Relevant for planning of health services Incidence: Relevant for exploring causal theories

Incidence & Prevalence:

Akhilesh Bhargava 34 Incidence & Prevalence Rate Type Numerator Denominator Morbidity Incidence New cases Total PAR* Mortality Incidence Deaths due Total population a disease or all causes Case fatality Incidence Deaths due to a No. of case of disease that disease Attack rate Incidence No. of cases of Total PAR for disease limited period Period Prevalence No. of Cases Total population prevalence New + Old * population at risk

Incidence increasing but prevalence decreasing:

Akhilesh Bhargava 35 Incidence increasing but prevalence decreasing

Interpretation::

Akhilesh Bhargava 36 Interpretation: # Disease duration is reduced and it is becoming acute, or # Disease becoming more fatal

Incidence stable but prevalence increasing indicates:- :

Akhilesh Bhargava 37 Incidence stable but prevalence increasing indicates:-

Slide 38:

Akhilesh Bhargava 38 1.Slow recovery (disease becoming chronic, drugs less effective) or, 2.Fatality reduced (potent drugs available, new drugs effective) or, 3.Immigration of cases from other area (for better facility available). Interpretation

Incidence maintained but prevalence declining means:- :

Akhilesh Bhargava 39 Incidence maintained but prevalence declining means:-

Slide 40:

Akhilesh Bhargava 40 # Recovery is becoming rapid, (may be a new drug identified is more effective) # Disease turns into a more fatal one (because of treatment failure, change in virulence, drug resistance) or, # Selective emigration of cases (to seek treatment elsewhere)

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