Australian Medical Sheepskin ARA0103 Aðferðafræði...

04/03/2008 Dr Andy Brooks 1

ARA0103Aðferðafræði Rannsókna

Fyrirlestrar 7 og 8Experimental design I/Tilraunasnið I

Australian Medical Sheepskin

quantitative/megindlegur


What is the problem?Hvað er vandamálið?

• Long term patients with bedsores.Langtímasjúklingar með legusár.

Dæmi... fyrst skref í rannsókn


What are the variables?Hverjar eru breyturnar?

• Independent variables:Óháðar breytur:– Bed mattress in use./Rumdýna í notkun– Frequency of patient turning.– Time in hospital.

• Dependent variables:Háðar breytur:– Number of pressure ulcers./Fjöldi legusára– Patient comfort.

• Using a sheepskin mattress cover might make the patient toohot.

Athugasemd: in student projects, start by considering only one independentvariable and one dependent variable.


What is the intervention?Hvað er inngrip?

• Medical Sheepskin…– http://www.healingfibres.com/– http://www.medicalsheepskins.com/index.html– http://www.classicsheepskin.com/medical.htm

• 2x frequency of patient turning/tvöfalda tíðni• Education/Menntun...

– If you can move yourself, change position every 15 minutes.– Eat properly/Borða rétt– ...

• Use pressure relieving supports...

olnbogi ökkli

http://www.classicsheepskin.com/medical.htm

http://www.medicalsheepskins.com/index.html

http://www.healingfibres.com/


Að bera saman gögn...

• Applying an intervention may beunnecessary if you can compare data fromdifferent wards/deildir, hospitals/sjúkrahúsor nursing homes/hjúkrunarheimili.

gögn hér

gögn hér gögn hérLess turning of patients?

More turning of patients?


State the null hypothesis H0Tilgreina núlltilgátuna H0

- áhrif er ekki til -

• A sheepskin mattress cover has no effecton the number of bed sores.

• Doubling frequency of patient turning hasno effect on the number of bed sores.

• Educating patients on how to prevent bedsores has no effect on the number of bedsores.

dæmi

Usually endeavour to reject the null hypothesis.Yfirleitt er leitast við að hafna núlltilgátuna.


State the alternative hypothesis HaTilgreina hin tilgátuna H0

- áhrif er til -

• A sheepskin mattress cover reduces thenumber of bed sores.

• Doubling frequency of patient turningreduces the number of bed sores.

• Educating patients on how to prevent bedsores reduces the number of bed sores.

Ef þú ert að nota t-próf, til dæmis, vonin er að segja:núlltilgátan er ekki rétt, frekar hin tilgátan hlýtur að vera rétt.

dæmi

hin tilgátan = aðaltilgátan

...minnkar fjöldi legusára


Select measurement toolsVelja mælitæknar

• Hjúkrunarfræðingur getur lagt samanlegusár beint.

• Rannsóknarmaður getur lagt samanlegusár að nota myndir sjúklings.

• Hjúkrunarfræðingur getur mælt hitasjúklings.

• A questionnaire can be used to assesspatient bed comfort.– rannsóknarmaður má skrifa sjálf– eða nota/breyta spurningalista sem er til

hjá t.d. http://www.medalreg.com/, …

http://www.medalreg.com/


ValidityRéttmæti

• Measurement tools are reliable if you getthe same answer by repeating themeasurement.– A nurse might fail to count a Stage I bed sore

on one ankle which was counted by anothernurse.

– Procedures have to be followed to measureblood pressure reliably.

• Measurement tools are valid if theymeasure what you want them to measure.– On a questionnaire, patients may say they are

too hot, but their temperatures measured usingthermometers suggest they were not too hot.

• Maybe they completed the questionnaires on a veryhot sunny day?

mælitæknar


Is the intervention potent?Er inngrip sterkur?

• A sheepskin mattress cover that is too thinmight not work.

• A video showing someone die in the finalstages of smoking-related lung cancer is amore potent intervention than aninformation leaflet/bæklingur.

• Toe-grasp training for 30 seconds everymonth is unlikely to improve balance.


Pilot study/Forathugun,Forrannsókn• If possible, conduct a pilot study with a small

number of people beforehand (4-6).• Pilot subjects should be similar to the subjects in

the actual experiment.• Pilot studies check on:

– Wording of questionnaires/Orðalag spurningalista.• How often are patients turned?

– Aldrei Stundum Oft Mjög Oft• Hvað þýðir stundum, oft, og mjög oft?

– Unclear instructions/Óvissar leiðbeiningar.• How often should the sheepskin mattress cover be washed?


Use a control groupNota samanburðarhóp

• The control group do not receive the intervention.• From an ethical point of view, the control group

should receive normal treatment/venuleg meðferð.• Using a control group is the only way to deal with

confounding variables.– age, sex, weight, surgery, mobility (gangandi?), ...

• Patient characteristics in the control group shouldbe similar to the intervention group.– Gengur ekki að bera saman 20-25 ára með 70-75 ára...

confounding variable/truflandi breyta


overview/yfirlit

Identify eligible subjects

Obtain informed consent/Afla upplýsts samþykkis

Þátttakendur Non-participants

Intervention group (I) Control group (C)

Participants Drop outs

Data collection

Þátttakendur

Data collection Safna gögn Safna gögn

Brottföll

Willing Unwilling

How many unwilling?

How many drop outs?How many drop outs?

Randomization/Tilviljunarval

Number of bed sores Number of bed sores

Randomized Controlled Trialhrein tilraunslembi-íhlutunarrannsókn

between subjects/milli hópa

eligibility criterion/úrtaksskilgreining


Randomization/Tilviljunarval

• Ideally, a third party allocates subjects to the control (C)or intervention (I).– Called allocation concealment/Kallað leynd úthlutunar.– Avoids experimenter bias/Forðast áhrif tilraunamanns

• An experimenter could allocate younger patients to the sheepskingroup (I).

• An experimenter could allocate only post-operative patients to thecontrol group (C).

• Subjects can be given a number and numbers can beselected at random using a computer program.

http://www.graphpad.com/quickcalcs/randomize1.cfm


• Obvious that the interventionreduces the number of leg sores.A t-test is not needed.

• Auglóst að inngrip minnkar fjöldilegusára. t-próf er óþarft.

• A t-test is needed. Is p < 0,05? Ifso, the difference is statisticallysignificant and the interventionreduces the number of leg sores.

• t-próf er þarft. Er p-gildi < 0,05? Efsvo, mismunurinn er tölfrælegamarktæk og inngrip minnkar fjöldilegusára

-> fjöldi legusára -> fjöldi legusára

tíðni tíðniInngrip Inngrip

Samanburðahópur

Samanburðahópur

Randomized Controlled Trial (RCT)overlap/skörun


Within subjects design/Innanhópasnið• Single subject/einn þátttakandi

AB design/AB snið.

Leggja saman legusár.Fyrr (A) og á eftir (B).

Ekki hægt að nota tölfræðipróf, svoer ekki hægt að álykta um þýði.

Líka ABAB, ABABAB snið.

A B

• An individual patient may be so unique and in need ofsuch help, that only they receive the intervention.

úrtaksstærð n=1

sample size/úrtaksstærð

Dr House...

04/03/2008 18

Within subjects design/ Innanhópasniðimbalanced/ójafnvægt

sheepskincontrolS2

sheepskincontrolS10

sheepskincontrolS9

sheepskincontrolS8

sheepskincontrolS7

sheepskincontrolS6

sheepskincontrolS5

sheepskincontrolS4

sheepskincontrolS3

sheepskincontrolS1apríl (I)mars (C)Subject/Þáttakandi

“paired” or “related” or “dependent” data-> nota parað t-próf

n=10

safna gögn lok mars og lok apríl


What if ?/Hvað ef?

• During the control period in March, thereare nursing staff shortages and patientsare turned only 50% of the time.– This would effect the measurements of bed

sores at the end of the control period.(skekkja)

• This within subjects design is said to beimbalanced and is fatally flawed.– gengur ekki að nota slíkt snið

04/03/2008 20

controlsheepskinS2

controlsheepskinS10

sheepskincontrolS9

controlsheepskinS8

sheepskincontrolS7controlsheepskinS6

sheepskincontrolS5

controlsheepskinS4

sheepskincontrolS3

sheepskincontrolS1

Apríl (I)Mars (C)Subject/Þáttakandi n=10

cross-overWithin subjects design/ Innanhópasnið

balanced/í jafnvægi

safna gögn lok mars og lok apríl


Within subjects design/Innanhópasnið

• By using the same subjects, there is bettercontrol of variability between subjects.

• But sometimes it is impossible for subjectsto receive both the control and theintervention.– Post-operative patients can be measured only

once. They are no longer post-operative afterreceiving either the control or the intervention.

post-operative/eftiraðgerðar-

advantage/kostur



• Sometimes the cross-over design leads toethical problems.– If the intervention works really well for an

individual patient, should they be taken of theintervention and put on the control ?

• Gengur ekki að hætta að lækna að nota inngrip!

• Patients are no longer blinded.Sjúklingar eru ekki lengur blindir.– They can realise that the use of the sheepskin

matress cover is the intervention.

siðfræðilegt vandamál



• Sometimes the cross-over itself is imbalanced.– Going from a sheepskin to a normal mattress is not

the same as going from a normal mattress to asheepskin.

• Sometimes a patient´s health status can changeover time.– Becoming post-operative as you are about to cross-

over between control (C) and interventiion (I).– Data for such patients may have to be left out.

• Data cleansing/Gagnahreinsun• Deletion or drop-out/Brottfall

ójafnvægt

heilsuástand breytast yfir tíma...


DæmiRegarding diastolic blood pressure, is there a statitiscally significant differencebetween men and women?Varðandi blóðþrýsting, er tölfræðilega marktækur munur til á milla manna ogkvenna?

Men 76, 76, 74, 70, 80, 68, 90, 70, 76, 80, 68, 72, 96, 80, 90, 72Women 76, 70, 82, 90, 68, 60, 62, 60, 62, 72, 68, 80, 74

Perform an independent t-test (2-tailed, unequal variances).Reiknaðu óparað/óháð t-próf (tvíhliða, ójafnar dreifingar).

What is the significance level? __________ (hvað er marktektarstig?)What is the effect size? __________ (hver er stærð áhrifa?)

The null hypothesis/Nulltilgátan:“There is no difference in diastolic blood pressure between men and women.”“Munur á blóðþrýstingi er ekki til á milli manna og kvenna.”

Between subjects design/Millihópasnið.


2,059538536t Critical two-tailp-gildi0,068009092P(T<=t) two-tail

1,708140745t Critical one-tail0,034004546P(T<=t) one-tail

the value of t1,907486345t Stat25df

null hypothesis0Hypothesized Mean Difference1316Observations

85,0769230869,71666667Variance71,0769230877,375MeanVariable 2Variable 1

t-Test: Two-Sample Assuming Unequal Variances

The value of p needs to be less than 0,05 for statistical significance.p-gildi < 0,05 þýðir að munur er tölfræðilega marktækur.

Equal sample sizes is better.Jafnar úrtakstærðir er betri.


more subjects?fjölga þátttakendum?

• p gildi er ekki minna en 0,05 en er lítið (0,07).• Munur á milli meðaltala er 6.• If there were more subjects, it might be possible to

detect a real difference.• A negative result in a statistical test might be because

you do not have enough subjects. The statistical testmight not have enough statistical power to detect thedifference between means.

• What if we have another set of measurements ofdiastolic blood pressure that are similar to the first?– Is it possible to obtain statistical significance?

Er hægt að ná tölfræðileg marktekt?

power of a statistical test/styrkur tölfræðiprófs


How many subjects?Hve margir þátttakendur?

• Ef þú ert að reikna út meðaltal ogstaðalfrávik, úrtaksstærð n hlytur að vera>=10. (segir Andy)

• n>=20 er betri• n>=30 er ennþá betri.• The greater the number of subjects the

greater the statistical power.

power of a statistical test/styrkur tölfræðiprófs

n>=30 meiri styrkur


• Ef n = 10, líkur á því að finna mun (sem er til) er ≈0,3.• Ef n = 20, líkur á því að finna mun (sem er til) er ≈0,6.

afköst

fjöldi í hverju hólfi sniðsins

Dæmi


Standard error of the meanStaðalvilla meðaltala

• The standard deviation of the sampling distribution of thesample mean is called the standard error of the mean.Staðalfrávik úrtakadreifingar úrtaksmeðaltals kallaststaðalvilla meðaltal.

nx

σσ =

The standard error of the mean gets less with a bigger sample.Staðalvilla meðaltala minnkar með stærra úrtaki.


RVLS Sampling Distributions

σ=5,00þýðisstaðalfrávik

S=2,27staðalfrávik úrtakadreifingar

24,255 =

n=5

normaldreifing




58,1105 =


n=10

normaldreifing



1255 =



n=25

normaldreifing


Central Limit Theorem/Markgildissetningin

• Even if the population distribution is skewed, the sampling distribution ofmeans is normal if the sample size n is large enough.

• If the population distribution is skewed and the sample size is small, youmust use non-parametric statistics. These tests have less statistical power.The equivalent of the parametric independent t-test is called the Mann-Whitney U test.

n=25

skewed/skekkt non-parametric test/stikalaust prófparametric test/stikapróf


DæmiDoes diabetes education improve scores on tests about diabetes knowledge?

68756760616272686577AfterEftir

59726460595570676275BeforeFyrr

Within subjects design/ Innanhópasnið

Null hypothesis/Nulltilgátan:“There is no difference in scores after a course about diabetes.”“Munur á stigafjölda er ekki til eftir nám um sykursýki”

paired or related or dependent t-test/parað t-próf

S1

Diabetes/Sykursýki

S2 S3 S4 S5 S6 S7 S8 S9 S10

Perform a paried/related/dependent t-test (2-tailed).Reiknaðu parað t-próf (tvíhliða).What is the significance level? __________ (hvað er marktektarstig?)What is the effect size? __________ (hver er stærð áhrifa?)

http://dtc.ucsf.edu/ Diabetes Teaching Center

http://dtc.ucsf.edu/


2,262157158t Critical two-tailp gildi0,004979545P(T<=t) two-tail

1,833112923t Critical one-tail0,002489772P(T<=t) one-tail

-3,692307692t Stat9df

null hypothesis0Hypothesized Mean Difference0,906845904Pearson Correlation

1010Observations33,6111111142,23333333Variance (s2)

67,564,3MeanVariable 2Variable 1

t-Test: Paired Two Sample for Means

p-gildi/marktektarstig er minna en 0,05. Við höfum tölfræðileg marktekt.

210/5,6

5,623,42

≈

≈


A two-tailed or one-tailed t-test?Einhliða próf eða tvíhliða próf?

• Diabetes education will not decrease peoples’knowledge about diabetes.– Í nám, þú lærir eitthvað...

• We can use the one-tailed t-test result.– The value of p is 0,00248.

• statistically significant/tölfræðilega marktæk

t=-3,69


Clinically significant effect?Klínísk marktæk áhrif?

• The paired t-test result (one-sided) showedthat diabetes education improvesknowledge about diabetes.

• En áhrifastærð er aðeins 3,2%.• Er það klínísk marktæk?• Do patients control their blood sugar levels

better as a result of only slightly improvedknowledge ?


Statistical significance does not imply clinical significance

• Reducing the mean number of bed sores by 1,0 meansone less bed sore for every patient.– Að meðaltali og er klínísk marktæk

• Reducing the mean number of bed sores by 0,1 meansone less bed sore every 10 patients.

• Reducing the mean number of bed sores by 0,01 meansone less bed sore every 100 patients.

• Reducing the mean number of bed sores by 0,001means one less bed sore every 1000 patients.– áhrifastærð er ekki klínísk marktæk

tölfræðilega marktæk er ekki sama sem klínísk marktæk

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