measures of association Flashcards

1
Q

what are measures of strength

A
  • risk ratio
  • odds ratio
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2
Q

what are measures of impact

A
  • risk difference
  • attributable fraction
  • number needed to treat
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3
Q

outcomes

A

disease, cure

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4
Q

exposures

A

risk factors

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5
Q

interventions

A

treatments

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6
Q

vet research compares association between

A

outcomes and exposure or intervention

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7
Q

odds

A
  • ratio of one outcome to that of an alternative outcome
  • calculated as number of outcomes divided by number on non outcomes
  • 51 boys in 100 births, ODDS any birth is a boy is 51/49=1.04
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8
Q

risk

A
  • probability an outcome will happen
  • calculated as number of outcomes divided by total number
  • 51 boys in 100 births; RISK of having a boy is 51/100=0.51
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9
Q

risk to odds

A

odds= risk/ (1-risk)

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10
Q

odds to risk

A

risk= odds/ (odds+ 1)

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11
Q

risk of disease in exposed and risk of disease in non exposed

A

just like normal thinking
either a/(a+b) or c/(c+d)

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12
Q

odds of disease in exposed or non-exposed

A

either a/b
or c/d

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13
Q

risk ratio

A

a/ (a+b) OVER c/(c+d)
Risk of exposed / risk of non exposed

ratio of the risk of disease in the exposed group to risk of disease in the unexposed group

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14
Q

odds ratio

A

a/b OVER c/d

which = ad/bc

the ratio between the odds of disease of exposed to odds of disease in unexposed group

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15
Q

RR interpretation

A

RR=1 then no exposure between exposure/ treatment and disease/cure

RR < 1 then exposure/ treatment reduces risk of disease/cure

RR>1 exposure/ treatment increases risk of disease/cure

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16
Q

risk ratio

A
  • provides an estimate of how many times more likely exposed/ treated animal experience disease/ cure compared w exposed/ treated animals
  • no units, ranges from 0-infinity
  • greater departure of RR from 1 the stronger/weaker the association
  • RR cannot be estimated in case-control studies, as these studies do not allow calculation of risks
  • recall its only relative; no indication to absolute risk, even if RR very high, even though overall disease risk is quite low
17
Q

odds ratio interpretation

A

OR= 1 no association between association between exposure/ treatment and disease/cure

OR<1 exposure/treatment reduces odds of disease/cure

OR> exposure/ treatment increases odds of disease/cure

18
Q

odds ratio

A
  • provides an estimate of the odds of disease in the exposed/ treated group to the odds of disease in the unexposed/ not treated group
  • OR can be calculated in any study design
  • no units, ranges from 0 to infinity
  • the greater departure of OR from 1 the stronger/weaker the association
  • symmetrical
19
Q

when odds ratio and risk ratio are v different what does that mean

A

disease is common

if they are v close then disease is NOT common

20
Q

measure of strength

A
  • risk ratio and odds ratio
  • measure the extent to which risk of disease/ success differs between exposed/ treated and unexposed/ not treated groups
  • they are useful for identifying risk factors, but do not provide info on abolsute risk
21
Q

measures of impact

A
  • risk difference, attributable factor, number needed to treat
  • express the impact (effect) of a risk factor on a disease/success
  • quantify the consequences in terms of absolute risk resulting from exposure to a risk factor
  • give an indication of the importance to the risk factor
  • these are the measures most common and most useful for clinicians
22
Q

risk difference

A
  • exactly what is sounds like, difference is risk of exposed minus risk of non exposed
  • a/(a+b)- c/(c+d)
  • reflects the increase or decrease in the disease/cure in the exposed/ treated group that is due to exposure/treatment
  • describes the absolute quantity of outcome measure that is associated w the exposure/treatment
  • clinical effects often expressed as RD
23
Q

number needed to treat

A
  • 1/RD
  • describes how many cases have to be treated to cure (or prevent) one additional case of the disease
  • ideal is 1, where every animal improves/ is cured w new treatment and none improve/ cured with control
  • so effective treatments have low NNT
23
Q

attributable factors

A
  • is proportion of successes for for the new treatment that is due to the new treatment
  • = RD/ risk in exposed group
    -= (RR-1)/ RR
  • = (OR-1)/ OR
  • percentage reduction or increase in risk of outcome in the exposed/ treated attributable to exposure/ treatment
  • relative measure of risk reduction