BioStats- Beehler Flashcards by Alisha Lindberg

What could you use to find the frequency of a disease in population?

Prevalence
Incidence
Attack Rate

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What would you use to determine how well a test differentiates sick from healthy people?

Sensitivity

Specificity

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What would you use to determine of those in a population who test as sick or healthy, how many are truly sick or healthy?

Predictive value

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What would you use to determine impact of a medicine/treatement?

Risk reduction/increase

Number-needed-to-treat/harm

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How do you determine prevalence?

Number of people with a disease at a specific point in time/ Number of people at risk for the illness at that point in time

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What are the two subtypes of prevalence?

Period prevalence- during a period of time

Lifetime prevalence- over the course of a lifetime

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A county in Minnesota has a population of 1,500. In 2013, 180 individuals were diagnosed with type 1 diabetes. Last year, 30 individuals were diagnosed with it. What was the incidence of type 1 diabetes in this population in 2014?

= 30/(1500-180) = 30/1320 = 0.023

The 180 are no longer new, so you need to remove the people who are no longer new

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A county in Minnesota has a population of 1,500. In 2013, 180 individuals were diagnosed with type 1 diabetes. Last year, 30 individuals were diagnosed with it. What was the prevalence of type 1 diabetes in this population in 2014?

= (180+30)/1500 = 210/1500 = 0.14

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How do you determine incidence?

Number of NEW people with disease during a time period/ Number of people at risk for illness during that time period

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What is attack rate?

Type of incidence used when nature of disease is acute & population observed for short period of time (eg outbreaks, specific exposures)

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How do you determine attack rate?

new cases/ # exposed

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What is secondary attack rate?

new cases/ (# exposed- primary cases)

measures person-to-person spread of disease after initial exposure

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Within a kindergarten class, 5 of 35 kids develop chicken pox during a 1-week period. In the next two weeks another 10 kids also come down with chicken pox. What are the attack and secondary attack rates of chicken pox in the classroom?

Attack = (5+10)/35 = 15/35 = 0.43

Sec. Attack = 10/(35-5) = 10/30 = 0.33

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How does duration of illness affect prevalence?

Longer duration = higher prevalence

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How does migration affect prevalence?

In-migration (ill = higher prevalence)

Out-migration (well = high prevalence)

Recovery & death = lower prevalence

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How does prevention affect incidence?

Tries to lower incidence

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Can changes in diagnostic criteria or reporting affect incidence?

YES!!!!!!

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When will prevalence be higher than incidence?

If disease is long term (like diabetes)

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What does it mean if prevalence is approximately the same as incidence?

Illness is acute (flu)

What is sensitivity?

The probability that a disease person will be identified correctly by a diagnostic/ screening test

True-positive probability or

True-positive rate

How do you determine sensitivity?

True positive/ (TP + FN)

A group of individuals who were exposed to Lyme disease were screened using a new test developed for early detection. Of the 344 screened, the disease was confirmed in 258. The new test detected 263 cases of Lyme disease, 32 of which were disconfirmed. What is the sensitivity of the new test?

= True positives / total # ill = 231/258 = 0.90

What is specificity?

The probability that a well (non-diseased) person will be identified correctly by a diagnostic/screening test (AKA true-negative probability)

How do you determine specificity?

TN/ (TN + FP)

What does having a high sensitivity mean?

- Errors on the side of over-diagnosing - Identify most or all possible disease cases - Most useful when under-diagnosing may lead to severe consequences (fast developing cancers)

What does having a high specificity mean?

- Errors on the side of under-diagnosing - Identify most or all well people - Most useful when over- diagnosing may lead to dangerous, painful, or unnecessary treatment?

What happens when you lower your cut off?

Lower specificity Raise sensitivity *double check this

What happens when you raise your cut off?

Raise specificity Lower sensitivity *double check this

What is predictive value?

Probability that a test will give the correct diagnosis

What does the predictive value depend on?

test sensitivity and specificity prevalence of the disease in the population being tested

How do you determine positive predictive value?

TP/ (TP + FP)

How do you determine negative predictive value?

TN/ (TN + FN)

How does high disease prevalence affect the PPV and NPV?

Higher PPV | Lower NPV

How does low disease prevalence affect PPV and NPV?

Lower PPV | Higher NPV

What is control event rate? How do you determine it?

Proportion of control group participants who have a bad outcome after “treatment” (e.g., placebo or no treatment) For example… If 10 of 30 control group participants become sicker, CER = 10/30 = 0.33 = 33% had adverse outcomes

What is experimental event rate? How do you determine it?

Proportion of treatment group participants who have a bad outcome after treatment (e.g., new drug) For example… If 4 of 30 treatment group participants become sicker, EER = 4/30 = 0.13 = 13% had adverse outcomes

What is absolute risk?

Absolute risk (aka “risk difference”): difference in risk of developing a disease or undesired outcome after treatment |CER – EER| *look for reduction or increase, focus on relationship between CER and EER

If CER > EER.... what happens to absolute risk?

Reduction (ARR) | Higher rate of adverse outcomes in CONTROL group

If CER

Increased (ARI) | Higher rate of adverse outcomes in TREATMENT GROUP

What is relative risk?

"risk ratio" Proportion of treatment group risk to control group risk EER/CER

After participating in an RCT of a new cancer drug, 10 of 30 control group participants become sicker and 4 of 30 treatment group participants become sicker. Did the new treatment reduce or increase absolute risk? By how much?

CER – EER = 10/30 – 4/30 = .33 - .13 = .20 = | 20% reduction

1 – RR = 1 – EER/CER = 1 - 13/.33 = 1 - .39 = .61 = | 61% reduction

Number needed to treat

Number of patients who need to be treated to get 1 additional patient a favorable outcome 1/ARR

Number needed to harm

Number of patients who, if they were treated, would result in 1 additional patient being harmed NNH = 1/ARI

How would you interpret NNT = 5?

For every 5 people treated, 1 more person would respond to the drug

How would you interpret NNH =3?

If 3 were treated, 1 more person would not respond compared to the control group