~randomized 

~assumptions of statistical significance are met

~dose levels are predetermined 

~masking/blinding reduces bias

~detailed info can be collected at baseline 

~expensive 

~ethics 

~time involved 

~ large number of participants 

~compliance 

~exclusions: may limit the ability to generalize the results

~they showed to be unreliable in early stages of the trial 

~exclusions/entry criteria: exclusions will improve internal validity but may diminish generalizability 

~px may refuse to take participate 

~poor understanding 

~confusion 

~ lack of finances 

Case control studies 

~rare diseases can be studied

~chronic diseases can be studied 

~less expensive

~less time consuming 

~smaller sample size

~can not directly measure risk 

~rare exposures

~recall exposure information(hard for px to remember what happened)

~selection bias 

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 

~begin with ppl who have a similar disease or prognostic factor 

~use prognostic factors

~outcomes are death or disability etc 

~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)

~ case fatality

~disease specific mortality

~5 year survival rate 

percent of patients surviving 5 years from

some point in the course of their disease

percentage of patients with the disease who die from it. 

example: 30 people developed SARS 26 died from it. 

~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

~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 risk

multivariable 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?

Risk Factors

   intrinsic or host factors – genetically determined (sex,blood type,)

Intrinsic host factors may be acquired  Specific immunity

extrinsic or environmental factors – may be biological, social or physical

Experimental- clinical trials 

Observational: cross sectional study (prevalence), case control and cohort study 

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. 

~measure relative risk directly

~ incidence of disease

~temporal relationship 

~rare exposures 

~cause and effect relationship 

~minimum bias 

~multiple exposures and outcomes 

~expensive 

~large sample size 

~time consuming 

~losses to follow up 

~changes over time (ie dx equipment)

~rare diseases 

Attributable risk (risk dfce)=

Iexposed - I non exposed

~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 drugs 

quit smoking, wear UV protection, regular eye exams, control diabetes

Nuclear

cortical 

sub capsular 

Age: ALL types

Diabetes: PSC

Steroids: PSC

Smoking: PSC and Nuclear

Sunlight: cortical and PSC

~Alcohol

  ~Estrogen  

~Hypertension

 ~Limited education

~Low body mass

 ~Lowheight

  ~Lowsocialclass

  ~ Low weight

         ~  Myopia   

Renal failure  

Rural residence

 Severe diarrhea or dehydration

~aspirin

~anti oxidants 

Age

smoking 

family hx 

IOP

Age

ethnicity (African american especially)

family hx

HTN

vasospatic disease

diabetes 

True or False 

Diabetes are 25 times more likely to become blind than the general population

1. prevalence 

2. incidence

Prevalence= # of ppl with the disease/ total population

P/(1-P) = I x D

(1-P): 1 - population free from disease

Sen= a/ a+c

spec= d/ d+b

Positive predictive values

= a/a+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)

~inherent dfces in individuals being tested

~variations in test or test conditions

~inconsistencies in the test procerdure or examiner

~Do not want to be labelled 

~cultural or language barriers 

~lack of available services in rural areas 

3 Top causes of visual impairment are?

1. uncorrected refractive error 

2. cataracts 

3. Glaucoma 

1. cataracts 

2. ARMD

3. Glaucoma

What is the most common cause of new cases of visual impairment among those over age 65

diabetes