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79 Cards in this Set
- Front
- Back
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proportion |
- division of 2 numbers - numerator is included in the denominator - quantities are of the same nature - proportions range between 0-1, or x100 (percentage) |
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ratio |
- division of 2 numbers - numerator not included in the denominator - can compare quantities of different nature |
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rate |
- division of 2 numbers w/ time included in the denominator - speed of occurrence of an event over time |
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cumulative incidence |
- proportion of a population that acquire a disease in a period of time - the probability of developing a disease # of new cases during a period/population at the beginning of the period |
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prevalence |
- proportion of a population that has a condition at a given point in time # of cases observed at a given time/total # of individuals at a given time |
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period prevalence |
# of existing cases + new cases developing during the study/total population |
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odds |
probability that an event will happen/probability that an event won't happen |
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relationship between probability and odds |
- probability and odds are more alike the lower the absolute P (risk) Prob = Odds/1+Odds Odds = Prob/1-Prob |
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risk difference |
cumulative incidence of Group 1 - cumulative incidence of Group 2 (a/a+b) - (c/c+d) |
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risk ratio (relative risk) |
cumulative incidence of Group 1/cumulative incidence of Group 2 (a/a+b) / (c/c+d) |
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odds ratio |
odds of disease in Group 1/odds of disease in Group 2 (a/b)/(c/d) |
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hypothesis |
statement of belief about population parameters |
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Type I error |
occur with a probability of alpha rejecting H0 when H0 is actually true as the probability of Type I errors increases, the probability of Type II errors decreases |
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Type II error |
occur with a probability of beta failing to reject H0 when Ha is actually true (Ha is true but we say it's not) as the probability of Type I errors increases, the probability of Type II errors decreases |
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confidence interval |
probability that a repeated study/test will get results within the confidence interval |
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predictive value positive |
probability that an individual with a positive test actually has the disease in question disease +, test +/total test + a/(a+b) |
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sensitivity |
probability that a test will be positive when applied to an individual who actually has the disease disease +, test +/total disease + a/(a+c) |
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specificity |
the probability that a test will be negative when applied to a disease free individual disease -, test -/total disease - d/(b+d) |
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false negative rate |
probability that an animal with the disease will have a falsely negative test result disease +, test -/total disease + c/(a+c) |
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false reassurance rate |
proportion of the negative test results that are actually false negative disease +, test -/total test - c/(c+d) ... OR 1-PVN |
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false positive rate |
probability that an animal without the disease will have a false positive test result disease -, test +/total disease - b/(b+d) |
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false alarm rate |
proportion of the positive test results that are actually false positives disease -, test +/total test + b/(a+b) ... OR 1-PVP |
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prior probability |
prevalence of the disease or condition in the population under study may be different in different populations |
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negative predictive value |
probability that an individual with a negative test actually is free of the disease non-disease indiv that test neg/total neg tests d/(c+d) |
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accuracy |
overall frequency of correct diagnostic classification, how close the answer is to the true value depends on prevalence unless sensitivity and specificity are the same |
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critical value |
study result that differentiates a positive finding from a negative finding |
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prevalence pool |
subset of population in the given disease state if an animal dies it is removed from the prevalence pool. if an animal recovers it exits the prevalence pool. |
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risk or propability |
probability that an event will happen/total cases |
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bias |
systematic difference of results from the truth - not due to random error prejudice in favor or against one thing, person, or group compared with another selection, information and confounding bias |
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sources of bias |
selection, misclassification, confounding |
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precision |
reflects the variability of the results, quality of being repeatable testing the same sample should give the same result increase precision w/ a larger sample size or by reducing the variability of measurements |
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confounding factors |
source of bias in a study related to both the risk factor of interest and the outcome variable |
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pattern recognition (Gestalt) |
looks like a duck, quacks like a duck... |
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method of exhaustion |
collect and sift through results of history, physical exam, lab tests, etc... expensive in time and lab costs, not effective |
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branching algorithms |
false positives and false negatives can lead to an inaccurate final answer |
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hypothetico-deductive method |
formulate short list of potential diagnoses selectively accumulate data to reduce length of list scramble/add to list until you get a good fit |
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power of the study |
probability of detecting a predefined clinically significant difference power = 1 - beta |
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significance level
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alpha or type 1 error probability of detecting a significant difference when treatments are equally effective risk of a false positive set at alpha = 0.05 as a standard |
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descriptive studies |
- looking at a population and describing it - also called hypothesis generating studies - case report, case series, survey/census - pros: use for generating hypotheses, informative for rare disease w/ few est. risk factors, characterize disorders - cons: can't study cause/effect, can't assess disease frequency |
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case report |
- descriptive study - describes rare disease in an individual or population - easy, low cost, low investigator control, no causal proof |
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case series |
- descriptive study - describes a series of similar cases - easy, low cost, low investigator control, no causal proof |
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survey or census |
- descriptive study - describes characteristics of a population - survey: collected from a sample of the population - census: collected from all members of the population - moderate difficulty, low cost, moderate investigator control, no causal proof |
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analytic studies |
- hypothesis testing or experimental studies
- observational or experimental
- evaluation of diagnostic tests, reviews |
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observational studies |
- analytic study - no individual intervention - treatment or exposures occur in "non-controlled" environment - individual can be observed: concurrently, prospectively, retrospectively |
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prospective study |
- observational, analytic study - looks forward in time - follows an exposure - examines future events |
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retrospective study |
- observational, analytic study - looks back in time - start with the disease - examines events that have already occurred |
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cross-sectional studies |
- observational, analytical study - determine disease and exposure at a single time point - often used to study conditions that are relatively common w/ long duration - moderate difficulty, moderate cost, low investigator control, low causal proof |
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case-control studies |
- observational, analytical study - data collected retrospectively - moderate difficulty, moderate cost, moderate investigator control, moderate causal proof |
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cohort studies |
- experimental, analytical study - compare individuals w/ known risk factor to individuals w/o the risk factor - evaluate risk of disease over a period of time - data usually collected prospectively - difficult, high cost, high investigator control, high causal proof |
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experimental studies |
- investigator "controls" the exposure - randomly assigned to groups - clinical trials most well known experimental design - ultimate step in causal hypothesis testing |
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randomized controlled trial |
- experimental, analytical study - subjects randomly assigned to group - treatment, control groups - prospective - difficult, expensive, high investigator control, high causal proof - GOLD STANDARD OF PROOF |
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validity |
the quality of being logically or factually sound |
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internal validity |
study population must be representative of the target population maximize internal validity by: - decreasing bias - sampling of target population - allocation to group |
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external validity |
ability to make inferences to populations beyond the target population can't be externally valid if not internally valid |
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selection bias |
- error occurs selecting individuals for a study - study population not representative of target population - minimize non-response to surveys, proper selection of comparison group |
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selecting a comparison group |
- pre-determined inclusion and exclusion criteria - observational study: case and exposure defns, random or other representative sample of comparison group - experimental study: random allocation to treatment/control group, sequential allocation |
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simple random sampling |
every animal has an equal chance of selection |
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systematic sampling
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every kth animal is selected |
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cluster sampling |
simple random sample of groups |
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stratified sampling |
population divided into mutually exclusive groups; random selection w/in each group |
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information bias |
- bias in measurement or gathering of data - i.e. data collected by a biased observer, recall bias, measurement, misclassification - have standardize protocol for data collection, use objective outcomes, use accurate and precise tests, use tests w/ high sensitivity and specificity, "blind" the person measuring |
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misclassification |
incorrect assignment of exposure and/or disease status non-differential or differential |
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non-differential misclassification |
frequency of classification errors is the same between groups - exposure misclassification is the same regardless of disease status OR - outcome misclassification is the same regardless of exposure status |
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differential misclassification |
frequency of classification errors is not the same between groups
- exposure misclassification differs based on disease status OR - disease misclassification differs based on exposure status |
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confounding bias |
"to mix up something w/ something else so that the individual elements become difficult to distinguish" - factor assoc. w/ both the exposure and the outcome - bias occurs when this isn't accounted for in design and/or analysis - restrict study to one level of confounder, collect data on potential confounders and account for it in analysis, randomize clinical trials, match on confounders in observational trials, stratify by level of confounder, account for using multivariable methods |
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health management considerations |
- animal welfare - food safety - herd economics - legality - strategic and tactical planning - change - needs to be part of general management |
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properties of data |
lag momentum bias dispersion |
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lag |
data is only available about an event after some time period has passed |
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momentum |
a parameter of interest changes slowly in response to real underlying change |
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dispersion |
the distribution of values for a parameter is such that the usual means of reporting may miss important features of herd behavior |
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qualitative data |
valuable in decision making and monitoring |
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cost of disease (dairy cow) |
cost of milk not produced - cost of feed not eaten = marginal cost of milk not produced |
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value of discarded milk |
milk discarded due to treatment is assumed safe to use to feed calves value of this milk is 1/2 the value of normal sale value |
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cost of a not pregnant cow |
fixed cost of $3.00/day assumed for an open cow |
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labor cost of a sick cow |
estimate an extra $16/hr labor of treating/handling sick cows |
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veterinary costs of a sick cow |
estimated at $100/hr for vet treatment of a sick cow |
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culling lame cows |
risk for culling increases two-fold with lameness varies slightly depending on etiology of lameness (i.e. digital dermatitis, foot rot, abscess, ulcer, complicated ulcer) |
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minimization of disease losses |
- reduce disease consequences: early detection & treatment, rational interventions - reduce diseases incidence |
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biological risk factors for neonatal diarrhea |
- calving area hygiene - calving assistance protocols - colostrum delivery - colostrum quality - calf housing - nutrition & feeding practices |
