statistics Flashcards

Question

Why are CI prefered to P values?

Answer 1

* Ci's relate to sample size * a range of values are provided * CI provide a rapid visual impression * CI have the same units as the variables

Answer 2

* **where a primary assumption is made that any difference seen occurred purely by chance** * collected data are then tested to disprove the null hypothesis * if the result is significant the null hypothesis is rejected on the basis that it is wrong

Answer 3

* 5% probability that the difference was seen was due to chance * often p = 0.05 * if the p value is \< 0.05 then this suggests the probability of the difference seen being due to chance is less than 5%

Answer 4

* **When a difference is found ** * **but in reality there is not a difference** * ie a false positive * null hypothesis is rejected incorrectly * this is the 5% of cases where the difference occured by chance * ie convincting an innocent man

Answer 5

* Reducing significant levels ( although this increases type 2 errors) * as reduce p values= reduces type 1 errors * but then bigger samples sizes are required to protect against type 2 errors

Answer 6

* **When no difference is found but in reality a difference does exist** * is a false negative * there fore the null hypothesis is falsely accepted * failing to convict a person guilty of the crime

Answer 7

* **Small sample size** * nb important to preform **power analysiss before** undertaking the study * protect against type 2 errors by statistical power * type 2 errors are common in ortho studies

Answer 8

* occurs rarely * **when the researcher correctly rejects the null hypotheiss but incorrectly attributes the cause** * ie the researcher misinterprets the cause and effect

Answer 9

* A method for determining the number of subjects needed to study in order to have a resonable chance of showing a difference if oen exists

Answer 10

* **is the probability of demonstrating a true effect or statistically significant difference** * 1-ß * expressed as a % * is the probability that the test will correctly reject the null hypothesis * if the power of the expt is low then there is a good chance the expt wil be inconclusive or give a type 2 error

Answer 11

1. **Size of the difference between the means** * the larger the difference the easier to detect a difference & \> the power 2. **Spread of the data** * the larger the spread, the less likely a difference will be detected 3. **Acceptable level of significance** * is the p value set 4. **sample size** * power increases with increasing sample size

Answer 12

* **The investigator observes rather than alters events.** * e.g. review of PE after THR

Answer 13

* **the investigator applies a maneovre and then oberves the outcome** * e.g. a surgeon may conduct a rct cf warfarin & heparin on the prevalence of DVT in pt with thr

Answer 14

* **Retrospective** study * **Prospective** study * **Cross sectional** study

Answer 15

* **The outcome of interest has already occurred and teh pt or cohort ( group) is followed forward in time from a point in the past**

Answer 16

* **follows pt or cohort forward in time** * _stronger than a retrospective study_

Answer 17

* **examines pts or events at 1 point in time without followup** * used when looking at the prevalence of a condition or desscribing distribution of variables

Answer 18

* type 1 = 0.05 * type 2 = 0.20 * ie power of 80%

Answer 19

* **increased risk of type 1 error ** * so **p value** may have to be **decreased** * or preform **Bonferonni correction**

Answer 20

* if one is testing n independent hypotheses, then one should use a significance level of 0.05/n. * e.g. 2 independent hypotheses then a result would be declared sig if P is less than 0.025

Answer 21

* Assumes data were sampled from **normal population** * observations must be **independent** * populations must have the **same variance** * can use **absolute difference between data points** * **increased power for a given sample size n** * rarely exists in orthopaedics

Answer 22

* **No assumptions** are made about origins of data * **no limitations on types of data** * **rank orde**r of values * **less likely to be significant** * **decreased power for a given n** * cannot relate back to parametric properties of data

Answer 23

* When there is a **pair of observations on a single subject ** * e.g. blood pressue before and after application of tournquet * aka students T -test

Answer 24

* Analysis of one way variance **ANOVA** * **it determines the proabability that 2 or more samples were drawn from the same parent population**

Answer 25

* used to **compare 2 random samples provided they both follow a normal distribution** * samples can be differing size but should be independent * ie no chance that a subject could appear in both of the groups being tested

Answer 26

* for **qualiatitve data** * used only on **actual numbers of occurances** ( frequencies) but not porportions/%/means or derived statistics * the test compares **distribution of a categorical variable in a sample with a distribution of a categorical variable in another sample** * it assess whether the observed data

Answer 27

* the **degree of association between 2 parameters**, with the correlation coefficient r being anywhere inbetween -1 and +1

Answer 28

* a measure of linear ie parametric association * if one pararmeter increases as the other does, then the correlation coefficient is positive

Answer 29

* A scatter plot * if a curve line is needed to express the relationship , then a more complicated measure of correlation must be used- Spearman's rank non parametric data

Answer 30

* regression is a straight line drawn over the scatted plot using the equation y=a+bx * the regression coefficient is the **direction** of the regression line

Answer 31

* shows **how one variable changes on average with another** * it can be used to find out what one variable is likely to be when the other is known * regression relationships may be linear, multiple or logistic

Answer 32

* **Indicates the amount of variance in the dependent variable is related to variance in the independent variable.** * ie if knee pain correlates with walking distance by r2= 0.6 then 60% of the variation in walking distance can be explained by variation in knee pain. The remaining 40% of variability is not explained

Answer 33

* _Level 1_ * **Meta-analysis/Systematic reviews of RCT ** * **Randomised controlled trials ** * _Level 2_ * **Prospective cohort study ** * **systematic review of level 2 trials** * _Level 3_ * **Case control study** * **Retrospective study** * **Systematic review of level 3 trials** * _Level 4_ * **Case series** * _Level 5_ * **expert opinon**

Answer 34

* Experts in their field has to say on a given subject * level 5 evidence

Answer 35

* Level 4 evidence * the outcomes of the group are reported, but there is **no comparison group/ control group** * **weak in relation to causation** * should act as a stimulus for more powerful studies

Answer 36

* **retrospective studies** where cases are gathered with a **certain outcome** and then **compared with controls that did not have the same outcome in order to look back at the effects of interventions /tx** * **quick and cheap to preform** * **limited by methological bias**

Answer 37

* 2 groups, **one** of which has undergone an **intervention** or tx are **followed up over time** in order to **compare outcomes** such as onset of disease or adverse effects * useful in identify incidence and established relative risk

Answer 38

* **Diagnostic access bias**- due to preselection * **expense** * **decreased validity** - due to loss to follow up

Answer 39

* Gold standard * **groups of patients are randomised to either recieve or not recieve an intervention or tx, and the outcomes are compared in a prospective manner**

Answer 40

* Randomisation * Generalisability * Sample selection * Outcome selection * Bias * Confounding factors * Masking/blinding * Ethics * Publication * Sequenetial analysis * Equvalence study

Answer 41

* it ensures that all prognostic variables both known and unknown will probably be distributed equally amongst the tx groups * this **avoids bias in treatment assignment**

Answer 42

* Valid * Reproducible * responsive to change * choice of outcome clinically relevant

Answer 43

* ie if a subject drops out during the study/tx the subject should still be included in the analysis * opposite is analysis per protocol/study

Answer 44

* Refers to flaws in impartiability that introduces systematic error into methodology and results in a study * Reduced by * **Randomization** * **masking** ( blinding) * meticulous attention to **study protocol**

Answer 45

* **experimental** * during either selection or tx * reduced by randomisation * **Observational** * errors in measurement or classification of disease * use of hip and knee scoring systems * **Patient bias** * **Publication bias**

Answer 46

* Independent variables that interfere with the drawing of statistically valid conclusions from a study * these factors may not be distrubuted equally between groups =\>**confounding bias**

Answer 47

* Matching e.g. age * Stratification

Answer 48

* A RCT in which 2 treatments are expected to have the same outcome * the research hypothesis is that there is a difference between the 2 groups aka alternative hypothess cf null hypothesis

Answer 49

* Meta - analysis- aim is to find **relevant evidence** from **several studies** in an unbiased manner and to apprasie each paper in all rct for metholoigcal quality. results are reported as a common estimate with confidence intervals * SR in that no common estimate of confidence intervals * cochrane collabration organises and publishes highly detailed systematic reviews in its database.

Answer 50

* "IATROGENIC" * **_Important_** conditon with known **Incidence** * **_Accepted_** and effective tx * **_Treatment_** and diagnostic facilities available * **_Recognizable_** latent and early symptomatis stages and consideration given as to whether early pick up at the latent stage leads to intevention adn whether intervention improves outcome * **_Opinions_** on who to tx are agreed * **_Guaranteed_** safety, sensitivity and specificity of test * **_Examination_** &/or tx are acceptable to pt * **_Natural history_** of condition known * **_Inexpensive_** tests, simple to preform * **_Cost effective screeening_** with a policy drawn up on whom to tx and it should be **_Continuously_** rolled out and repeated at intervals

Answer 51

* Is the study of **frequency** and **cause of diseases** in human populations

Answer 52

* **Is the rate of occurance of new disease in a population previously free of disease** * the rate is found by dividing the number of new cases in the study period by the number of individuals at risk at the beginning of the study period

Answer 53

* **Is the frequency of a disease at a given time** * found by dividing the no of patients with the disease by the sum of the number of patients with the diease and number of patients at risk

Answer 54

* **The ability of the test to exclude false negatives** * ie the ability of the test to pick up all causes of disease * **no true positives / true positive + false negative**

Answer 55

* **Ability of the test to exclude false positives** * ie ability to exclude the disease * no of true negatives/ true negatives + false postives

Answer 56

* **Is the probability that a subject who tests positive is truly positive** * ie the PPV indicates the significance of a positive test * PPV= true positive/ true positive + false positives

Answer 57

* is the probability that a subject who test negative is truly negative * is the NPV indicates the significance of a negative test * NPV= True negative/ true negative + false positives

Answer 58

* **Gives an idea of how often a test is correct** * True positive + true negatives/ True positive + False positive+ True negative + false neagative

Answer 59

* Used in case control studies * **is the ratio of the odds that an event will occur in one group to the odds that the event will occur in the other group.** * OR = (c/d)/( a/b) * = cb/ad

Answer 60

* **Success rate of tx group - sucess rate of control/ success rate of control** * succes rate of tx = c/ (c=d) * Success rate of control group = a/(a+b)

Answer 61

* is the extent to which a test or outcome measure actually measures what it purports to measure * test have to be **precise** ( consistency of repeated measures ) and **accurate** ( represent what they mean to represent)

Answer 62

* **Construct validity** * the extent to which a measure corresponds to theoretical concepts or constructs concerning the phenomenon of interest * **Content validity** * the extent to which a measure represents the domain of interest * **Criterion or concurrent validity** * correlating scores on a new instrument or test with external criteria known or believed to measure the attribute

Answer 63

* The random error of a measure * important to consider reliability within the same assessor **intra-observer** and different- **interobserver**

Answer 64

* Involves adjusting the observed proportion of agreeement in relation to the porportion of agreement expected by chance * Used for categorical data * a value of 1.0= complete agreement * a value of 0 = agreement can be explained purely by chance * a negative value= systematic disagreement * can be **weighted or unweighted** * weighted kappa statistics allow for the measuring of observer agreement in rank scales taking into account agreeement by chance and bringing the magnitude of disagreement into calculation

Answer 65

* Is the study in which the **outcome of an intervention is plotted over time which allows for variable dates of entry and for patients to be followed up for different lengths of time** * analysed continuously = **actuarial method** * times at failure= **Kapalan -Meier product limit method** * combo of both= **life table analysis**

Answer 66

* **Define end point / outcomes** * sucess * failure * death * revision * For each joint replacement **the number of joints** being followed and the **no of failures** are determined for **each year** after operation * at each time point the **no of pts at risk, the no of failures, no of pts withdrawn** ( death/LTFU) recorded. * pts who complete trial and deaths= successful withdrawals / censored data/ non endpoints * these don't count as failures and only affect no of pts at risk * each year the no of pts at risk calculated = as no of pts at beginning of year- 1/2 the no of withdrawals

Answer 67

* **no of failures/ no of pts at risk during period**

Answer 68

* Cumulative estimated survival = **100%- cumulative proabability of failure**

Answer 69

* By cumulating the success rate for all previous years and year in question

Answer 70

* Survival rates annually for life table analysis * Recalculated every time a failure occurs for a kaplan meier * the steps in the graph represent failures at each time point

Answer 71

* **IS the cumulative estimate of suvival plotted with 95% CI** * upwards blips of solida circles are used to represent censored data on graphs * when reporting survival analysis on emust inlcude 95% CI , best and wirse case scenerios adn no of pts left at longer follow-up

statistics Flashcards

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