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Measures of Disease Frequency

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welcome in this session we are going tosee some measurements we commonly use tomeasure that this is frequency before wego in for appropriate measurement ofdisease frequency we need to understandcertain concepts like a population atrisk the population at risk are theportion of population that issusceptible to a disease that can bedefined on the basis of demographic orenvironmental factors now let's ratherlook at some example the population atrisk of developing carcinoma of cervixis a female population and in the agegroup of 30 to 70 years a population atrisk of hepatitis B are thoseindividuals who are at risk ofdeveloping hepatitis B but what negativenow lets leather look at a measure whichis very often huge in epidemiology inany Health Research that's calledprevalence what do you mean byprevalence the prevalence is the numberof existing cases both old and new in adefined population at a specified pointof time P is equal to the number ofpeople with disease at specified timeand that's divided by the population atrisk at the specified time and thatquantity is multiplied by a factor by 10or a 10 power and in order to make thatas a suppose you know your population atrisk is very large and you have numberof cases who are small you will get avalue P as 0.001 so in order to eithermake it into your own number youmultiplied by thousand or 10,000 or100,000 depending on whatthe value P you get. So in substudies thetotal population used as anapproximation if data on population atrisk is not available because with thephilosophy that everyone in thepopulation are at risk of developingparticularly disease again thisprevalence are broadly classified intosay two different categories we callthis one as a point prevalence andanother one as a period preference butpoint through prevalence what we mean isthis measures the frequency of diseaseat a given point of time it's like asnapshot this applies when the data hasbeen collected at one point in time it'sdenoted as P is equal to C by n where Cis the number of observed cases at thatparticular point of time T and n is thepopulation size at time point e theexample of a point prevalence is supposeif there are 150 children in a schooland you are screening them forrefractory errors at a particular pointof time T and you find 15 children theyrequire classes they have problem thenthe prevalence of refractory errors is15 divided by 150 which is equal to 10and called a 10% of the school childrenthey have refractory errors or the pointprevalence of refractory error in thisparticular school is 10% the periodprevalence is that measures thefrequency of disease over some timepoint over a period of time this applieswhen the data has been collected over aperiod of time and it is denoted as P Pwhich is equal to C plus I divided by nwhat is C? C is the number of prevalentcases at the beginning of that timeperiod and I the incident cases that isthe new cases that develop during theperiod of your survey and these wholethere distort some of these two aredivided by nwhich is the size of the population for thissame time period point and an example ofthis period prevalence is in or take ascenario of a population of 150 percentsand you follow-up for one year at thebeginning of your survey there 25 haddisease of interest in the big andanother 50 new cases developed duringthe year and we calculate the pointprevalence at the start of the periodand period turbulence over the period oftime point prevalence is given by C by nthat is 25 by 150 or 0.1 7 or 17% periodprevalence is 25 plus 15 that is 40 or150 it comes 0.27 that is 27% so nowthere are several factors that influencethe value of prevalence the number ofnew cases the duration of illnesssuppose if the duration of illness isshort the prevalence is radius saysuppose somebody gets the disease andimmediately you know is there a fuse orit dies then it goes out of ourcalculations because when we go thereare no disease passes so the prevalentof sudden infant deaths right timeswould be even zero because when you goif there are no deaths at hand they'reall all the children had that diseasehad died so there are no cases at thatparticular point of time so yourproblems may be zero and the diseases ofvery long duration the problems you knowit goes on suppose certain chronicdiseases are a lifelong diseases it canaccumulate to build up a very largeprevalence okay so we have to rather becareful what causes an increase ordecrease in prevalence and increase inprevalence could be caused by longduration just low cure rate or low casefatality and there are more number ofnew cases that have come back or thereare immigration population patients whohave with particular disease if theyimmigrate into a particular area theycould rather enhnace the prevalence Prevalencecould also be increased if there is anew improved detection mechanism you tryto Renault you detect more cases becauseyou have more sensitive tests in yourhand and proponents could also increaseif there are healthy people going out ofa particular region so that yourdenominator is no and your numeratorthey all and healthy or diseased peopleare there and so your prevalence mayincrease The decrease in the prevalencecould happen exactly no the oppositecauses the shorter duration high curerate high case fatality the decrease innew cases emigration of patientsimproved cure rates and emigration ofhealthy people all these could bringdown the prevalence so to conclude thechanges in prevalence may have manycauses under difficulty interpret sobetter we need to have a checklist ofall these items and then look at themall before we write rather say that theprevalence has increased or decreasedfor a period of time in a particularregion what are the uses of privilegeddata provenance of data is used toassess health cabinets it's very usefulin planning health services because itmeasures the burden of disease and itmeasures the occurrence of conditionswith gradual onset and prevalence isvery useful in the study of chronicdiseases another important measure inepidemiology is the incidence theincidence is defined as the number ofnew cases in a given period in aspecified population that is time is animportant component rather we measure itby day or month or here that must bespecified when you are mentioning theincidence this measures the rapiditywith which of the speed of occurrencewith which new cases are occurring in apopulation this can be expressed inabsolute numbers in terms of accumulatedincidence or in terms of inSiddhant density let's rather look atthe different component aspects of theincidence let's rather take theaccumulated incidence bus the cumulativeincidence is CI is a number of new casesdivided by population at risk at thebeginning that's multiplied by a factorof 10 this is known as attack rate andit assumes that the entire population atrisk at the beginning was followed upfor the time period of observation nowlet's say the look at what do we mean byrisk the risk is the probability that anindividual will experience a healthstatus change for a specified follow-upperiod this assumes that the individualdoes not have disease at the beginningand died from other causes duringfollow-up this corresponds toaccumulated incidents now let's look atthe other measure which is calledincident density denoted as I D theincident density is the number of newcases divided by total person time ofobservation and that's multiplied by afactor of 10 this is also known asincidence rate this reflects moreexactly the person time observed anduses of this incident data is thisincident data describes trends indiseases whether the trend over a periodof time how the particular diseasechanges and it evaluates impact ofprimary prevention programs the dynamicof incidence and prevalence can bedepicted with the diagram like this seethere are new cases pouring in there arecases that are growing and going outbecause they are dying or they'regetting cured and so the in but theincident cases are the new cases and thecases which are remaining in the tab orthe prevalence casesso the relationship between theprevalence and incidence could be theprevalence depends on the incidence andthe duration of disease and it's denotedas prevalence is equal to incidence intoduration P is equal to I into D changein prevalence from one time period toanother may be the result of changes inthe incidence rates changes in theduration of disease or could be both sonow let's look at the patterns ofincidence and prevalence high prevalenceand low incidence there are diseaseslike diabetes mellitus no problems andhigh incidence are the examples arecommon coldnow let's define a term called the casefatality in case fatality the play-inrelation it's the relation to the numberof deaths from a disease to the numberof cases how many cases you have and howmany of them they died it reflects theseverity of the case this can beexpressed as a proportion or a ratio andnot as a rate though it's often referredto as case fatality rate so let'ssummarize this this is basic diseasefrequencies that we are using one is theprevalence and prevalence is a staticmeasure taken at a point in time or overa period of time if you take it at apoint in time it's called a pointprevalence or a period of time it'scalled a period prevalence incidence isa dynamic measure taken over a certaintime and the mortality is calculatedusing population denominators to reflectthe burden while the case fatality iscalculated using cases as denominatorsto reflect severity and these are allthe measures of disease frequency thankyou very much for your attention