the number of patients that would have to be treated in order to prevent one occurrence.
numbers to treat= 1/ absolute risk reduction
Which type of study should be used for rare or chronic diseases?
Case control studies
what are the advantages of a case control study ?
~rare diseases can be studied~chronic diseases can be studied~less expensive~less time consuming~smaller sample size
what are the disadvantages of a case control study ?
~can not directly measure risk~rare exposures~recall exposure information(hard for px to remember what happened)~selection bias
The odds that a case is exposed divided by the odds that a control is exposed
Odds ratio case control
To estimate relative risk with an odds ratio what are the two assumptions that must be fullfilled:
1. The frequency of disease in the population must be small (annual incidence <1/100)2. case control must be well designed
what is the most likely thing to cause microbial keratitis?
~extended use of soft contact lenses
The prediction of the future course of the disease following its onset
studies of prognosis are similar to cohort studies except?
~begin with ppl who have a similar disease or prognostic factor~use prognostic factors~outcomes are death or disability etc
What is the sequence of natural history of disease?
~stage of susceptibility (Have risk factors)~presymptomatic stage of disease(no signs or sxs)~stage of clinical disease (recognizable signs or sxs)~stage of disability (any limitations on persons ability)
what are the rates used to describe prognosis?
~ case fatality~disease specific mortality~5 year survival rate
percent of patients surviving 5 years fromsome point in the course of their disease
5 year survival rate
percentage of patients with the disease who die from it.example: 30 people developed SARS 26 died from it.
number of people per 10,000 (100,000) population dying of a certain disease
Disease specific mortality
What types of bias are found in cohort studies ?
~susceptibility bias (occurs when groups of patients with and without the prognostic factor under study differ in other ways)~survival cohort: available or survival cohorts may represent a bias view(they survived when others did not)~migration bias: px leaving the study to either enter the study under another group or dropping out~measurement bias: if px in one group has a better chance of having their outcomes detected than a px in the other group
what are some methods for controlling selection bias?
~Randomization–assign patients to groups in a way that gives each patient an equal chance of falling into one or the other group.
Restriction–limit the range of characteristics of patients in the study.~Stratification: compare rates with in subgroups (strata) with other wise similar probablility of the outcome. Simple adjustment (standardization): mathematically adjust crude rates for one or a few characteristics so that equal weight is given to a strata of equal riskmultivariable adjustment: adjust for dfces in a large number of factors related to outcome using mathematical modeling techniques Matching–for each patient in one group, select one or more patients with the same characteristics (except for the one under study) for a comparison group.!
What is leading cause of new cases of blindness in people 20 to 74 years of age?
Factors that cause an increased risk of becoming diseased are?
Risk Factorsintrinsic or host factors – genetically determined (sex,blood type,) Intrinsic host factors may be acquired Specific immunityextrinsic or environmental factors – may be biological, social or physical
What are 2 dft study designs and give examples of each
Experimental- clinical trials Observational: cross sectional study (prevalence), case control and cohort study
What is the only type of study that incidence/risk can be measured directly ?
Cohort study ~ppl begin w/o the disease and then are monitored and documented to see if they develop the disease/condition have exposure to a certain factor.
What is the relative risk ratio?
The ratio of the incidence of disease in those exposed to incidence of disease in those non exposed
What are the advantages of a cohort study?
~measure relative risk directly~ incidence of disease~temporal relationship~rare exposures~cause and effect relationship~minimum bias~multiple exposures and outcomes
What are the disadvantages of a cohort study?
~expensive~large sample size~time consuming~losses to follow up~changes over time (ie dx equipment)~rare diseases
The risk of disease attributable to exposure.
Attributable risk (risk dfce)=Iexposed – I non exposed
The incidence of a disease in the population associated with the occurrence of a risk factor
population attributable risk
The fraction of a disease in the population that is attributable to the exposure to a risk factor
Population Attributable fraction
how many times more likely are exposed people to become diseased relative to non exposed people
Relative risk (risk ratio)
What are the risk factors associated with diabetic retinopathy ?
~insulin depedent diabetes (higher risk)~duration of diabetes(longer you have had it the higher the risk)~higher mean glycosylated hemoglobin
what are the 4 major eye diseases? what increases the risk of these diseases?What can be done to prevent them?
Cataracts, ARMD, glaucoma and diabetic retinopthay smoking, sun exposure??, age, family hx, some drugsquit smoking, wear UV protection, regular eye exams, control diabetes
What % of px were unaware that they had eye disease?
48.6% (almost half)
True or false if you have good best corrected VA then you will not have eye disease?
what is the most common cause of blindness world wide?
What are the 3 main types of cataract?
Which type of cataract is caused by the following factors?
Age: ALL typesDiabetes: PSCSteroids: PSCSmoking: PSC and NuclearSunlight: cortical and PSC
Factors associated with Cataract – Possible risk factors
~Alcohol ~Estrogen ~Hypertension ~Limited education ~Low body mass ~Lowheight
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Lowsocialclass ~Low weight ~Renal failure Rural residenceSevere diarrhea or dehydration
What are the possible protective factors for cataracts?
What are the 3 definite risk factors for ARMD?
The risk factors for POAG
IOPAgeethnicity (African american especially)family hxHTNvasospatic diseasediabetes
True or FalseDiabetes are 25 times more likely to become blind than the general population
what are the 2 basic measures of disease occurrence?
What portion of the population has a disease at a specific time?
Prevalence= # of ppl with the disease/ total population P/(1-P) = I x D (1-P): 1 – population free from disease
The frequency of new cases that occur in a given time period
Incidence rate = # of new cases/ person years at risk
The proportion of healthy individuals that get a disease over a certain period of time?
Cumulative incidence = new cases/ # of ppl at risk during the study
Probablity that a px has a condition could be determined by which measure?
Prevalence and predictive values
What measure can be used to “predict the future”
What measure would we use to make comparisons about frequency of disease in ppl exposed to a risk factor and ppl not exposed
Is a measure of how closely a series of obervations of exactly the same thing match one another
Is a measure of how closely observations correspond to the “truth” of the actual state of affairs
What are the formulas for sensitivity and specificity?
Sen= a/ a+c spec= d/ d+b
the probablity of disease in a px with an (abnormal) positive test result
Positive predictive values= a/a+c
The probability of not having the disease when px has a normal test result.
Negative predictive value= d/d+c
The probability of that test result in people with the disease divided by the probability of the result in people who do not have the disease~(how many more times likely is a test result is to be found in diseased compared to non diseased person
Likelihood ratio=(a/a+c )divided by (d/d+b)
What three factors can affect the reliability of the test ?
~inherent dfces in individuals being tested~variations in test or test conditions~inconsistencies in the test procerdure or examiner
legally or registered blind is when?
6/60 20 deg of field
What are barriers to being registered as legally blind?
~Do not want to be labelled~cultural or language barriers~lack of available services in rural areas