Measures of Disease Frequency - The INCLEN Trust

MeasuresofDisease Frequency• Rates, Ratios, and Proportions• Incidence versus Prevalence• Risk• Rate• Risk versus Rate• Mortality• Risk/Rate Adjustment

Hypothetical data on the frequencyof hepatitis in two citiesLocation New cases Year PopulationCity A 58 2002 25,000City B 35 2001-2002 7,000

Hypothetical data on the frequencyof hepatitis in two citiesLocation New cases Year PopulationCity A 58 2002 25,000City B 35 2001-2002 7,000Annual rate of occurrence of hepatitis:City A: 58 / 25,000 / 1 year = 232/10 5 / yearCity B: 35 / 7000 / 2 years = 17.5 / 7000 / 1 year= 250/10 5 / year

Rates, Ratios, Proportions• Three general classes of mathematicalparameters.• Often used to relate the number of cases ofa disease or health outcome to the size of thesource population in which they occurred.

5101520252 Masks2.1 “mask”The basic mask delineates ocean, grounded ice sheet and ice shelf regions of Antarctica.The mask is derived from the MOA (Mosaic of Antarctica) coastline shapefiles(Haran et al., 2005; Scambos et al., 2007). The ice thickness dataset (described laterin Sect. 3.1.3) is composed of two separate datasets: the grounded ice sheet thickness,largely from BEDMAP, and ice shelf thickness, derived from the surface elevationand an assumption of hydrostatic equilibrium. In order to create a smooth join betweenthe grounded and floating ice thickness datasets, a number of iterations are carriedout where the mask (more specifically, the grounding line location) is modified to ensurethe best join between the two datasets. Small changes are made away from icestream grounding line areas, allowing for a one grid cell width smoothing boundary(see Sect. 3.1.3). Larger changes are made at a number of ice stream grounding zonelocations, where the MOA grounding line is believed to be inaccurate. The areas ofmajor change are discussed in Sect. 2.3 below.Three islands were found to be missing from the original MOA dataset (Smyley Islandand Case Island at the western end of George VI Ice Shelf, and Sherman Islandincorporated in the Abbott Ice Shelf), these were added by digitising their extent fromthe original MOA image. The mask values are given in Table 2.The mask was then modified to make it “modelling friendly”, i.e. by removing smallice shelves (1 or 2 cells), and islands only one grid cell wide.2.2 “mask+”The purpose of the “mask+” mask is to provide extra information on the ice sheet,beyond that provided in the basic mask. It indicates terrestrial regions that are currentlyice free and ice stream regions, here defined by velocities over 250 m yr −1 .199Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |ESSDD3, 195–230, 2010Antarctic dataset forhigh resolutionnumerical ice sheetmodels (ALBMAP v1)A. M. Le Brocq et al.AbstractTitle PageInstrumentsData Provenance & StructureTables◭◭BackFigures◮◮CloseFull Screen / EscPrinter-friendly VersionInteractive Discussion

Rate• A measure of how quickly something of interesthappens.• Expressed as:x 10yExample: The number of new cases of Parkinson’sdisease which develops per 1,000 person-years offollow-up.# of new cases of Parkinson's disease1000Total time disease -free subjects observed• Time, place and population must be specified foreach type of rate.n

Measures of Disease Frequency• Incidence (I): Measures new cases of adisease that develop over a period of time.• Prevalence (P): Measures existing cases of adisease at a particular point in time or over aperiod of time.

Prevalence vs. Incidence• Prevalence can be viewed as describing a pool ofdisease in a population.• Incidence describes the input flow of new cases intothe pool.• Fatality and recovery reflects the output flow fromthe pool.

House Guest ExampleGuests arrive 2 per day and stay 5 days.How many guests in the house after a week?Answer: 10 (P=10, I=2, D=5)

RiskProbability that an individual with certaincharacteristics such as:AgeRaceSexSmoking statuswill experience a health status change over aspecified follow-up period.Assumes:Does not have disease at start of follow-up.Does not die from other cause during follow-up.

Risk0 ≤ RISK≤ 10% ≤ percentage ≤ 100%Specify risk periodExample:The 10-year risk that a 45-year-old male willdevelop prostate cancer is 5%.

Cumulative IncidenceCI INI = # of new cases during follow-upN = # of disease-free subjects at start of followupMeasures the frequency of addition of newcases of disease and is always calculatedfor a given period of time.

Example

Cumulative Incidence• Most common way to estimate risk.• Always a proportion• Assumes a fixed cohort• For brief specified periods of time, e.g. an outbreak,commonly called an Attack “Rate”- Beware of attrition- Formula does not reflect continually changingpopulation size for dynamic cohorts.- Does not allow subjects to be followed for differenttime periods.

Rate• Describes how rapidly health events are occurringin a population of interest.• In Epidemiologic studies, we typically measure theaverage rate at which a disease is occurring over aperiod of time.• Example:50 new cases per 10,000 person-yearsInterpretation:An average of 50 cases occurs for every 10,000years of disease free follow-up time observed on acohort of subjects.

RateWhich of these describes a rate?1) 5 new cases per 100 person days2) 40 miles per hour3) 10 new cases out of 100 disease-freepersons4) 60 new murders per year5) 60 deaths out of 200 lung cancer patients

Incidence RateIR IPTI = # of new cases during follow-upPT = total time that disease–free individuals in thecohort are observed over the study period.Synonyms: hazard rate, incidence density rate.Measures the rapidity with which new cases areoccurring in a population

ExampleHypothetical cohort of 12 initially disease-free subjectsfollowed over a 5-year period from 1990 to 1995.

Example, cont.IRIPT525PY0.2020 new casesper 100 person-yearsStudy questions:1) Is the value of 0.20 a proportion?2) Does the value of 0.20 represent the riskof developing disease?

Parkinson’s Disease

Confusing Risk with Rate• The term “Rate” is often been usedincorrectly to describe a measure of risk.e.g.,• Attack Rate• Death Rate• Case-Fatality Rate• When reading Epidemiologic literature, oneshould be careful to determine the actualmeasure being reported.

Prevalence• Measures existing cases of a healthcondition• Primary feature of a cross-sectionalstudy• Two types of Prevalence• Point prevalence• Period prevalence

Point PrevalenceP CNC = # of observed cases at time tN = Population size at time tMeasures the frequency of disease at a givenpoint in time

Point PrevalenceExampleSuppose there are 150 individuals in apopulation and, on a certain day, 15 areill with the flu. What is the estimatedprevalence for this population?P 15 15010%

Study Questions1) Is point prevalence a proportion?2) Suppose 13 people from a population of size406,245 had a particular disease at time t. What isthe point prevalence of this disease at time t?3) Which of the following expressions is equivalent tothe point prevalence estimate of 0.000032?a. 3.2 per 1,000b. 3.2 per 100,000c. 32 per 100,000

PrevalenceUseful for:• Assessing the health status of apopulation.• Planning health services.Not Useful for:• Identifying risk factors

Period PrevalencePPCN• C = the # of prevalent cases at the beginning of thetime period.• I = the # of incident cases that develop during theperiod.• N = size of the population for this same time period.I

ExampleSuppose we followed a population of 150 persons forone year, and 25 had a disease of interest at thestart of follow-up and another 15 new casesdeveloped during the year.1) What is the period prevalence for the year?2) What is the point prevalence at the start of theperiod?3) What is the cumulative incidence for the one yearperiod?

ExampleSuppose we followed a population of 150 persons forone year, and 25 had a disease of interest at thestart of follow-up and another 15 new casesdeveloped during the year.1) What is the period prevalence for the year?PP = (25 + 15) / 150 = 0.27 or 27%2) What is the point prevalence at the start of theperiod?P = 25/150 = 0.17 = 17%3) What is the cumulative incidence for the one yearperiod?CI = 15/125 = 0.12 = 12%

Mortality RiskDisease-Specific Mortality RiskAll-causes Mortality RiskCase-Fatality RiskCI IN

Mortality RateDisease-Specific Mortality RateAll-causes Mortality RateCase-Fatality RateIR IPT

AgeAdjustment