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Samenvatting Practicum 2
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Samenvatting colleges en online modules. Wetenschappelijke scholing deeltoets 2, semester 2.
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Vragen practicum 2 - elearningModule 7.B:
Wetenschappelijk onderzoek 2 groepen:
- Experimentele groep
- Controle groep
Deze groepen zijn niet verbonden —> onafhankelijke groepen
T-test kan hierbij uitgevoerd worden
X1 − X2 s12 s22
t= SE(di f f ) = (SE(X1 ))2 + (SE(X2 ))2 = +
SE(di f f ) n1 n2
Als de variantie is groep 1 gelijk is aan groep 2 kan een pooled estimate formule gebruikt worden:
(n1 − 1) ⋅ s12 + (n2 − 1) ⋅ s22
sp =
n1 + n2 − 2
De wegingsfactoren voor de gekwadrateerde standaarddeviaties (de varianties) zijn de
vrijheidsgraden van elke groep (n - 1).
Test om te controleren of de varianties gelijk zijn —> Levene’s test:
- p < 0,05 dan zijn de varianties significant (H0 verwerpen)
- p > 0,05 dan is het gelijk (H0 handhaven)
Bij pooled test (varianties zijn gelijk) bij 2 groepen, df = dfA (nA-1) + dfB (nB-1) —> nA+B - 2
Als varianties niet gelijk zijn bij 2 groepen, heeft df meer complexe formule
Voorwaarden pooled test (independent sample t-test):
- Observaties zijn onafhankelijk
- Data is normaal veredeld
- Varianties in groepen zijn gelijk
, 1. Fill in the missing numbers. Round to two digits.
A group of women were born in the south of the country ,another group were born in the
north. We would like to test if the average heights for the groups are equal. Use the SPSS
output in Figure 7.6 to answer the following questions (use α = 0.05 for the tests)
Give the 95% CI for the mean height of women from the north and give your interpretation of
it: […… ; ……]. We are ……% confident that the real population mean height of women from
the north is between …… and …… cm.
Antwoord: 171,2128 ± 2,004 ⋅ 0,699 = [169,81; 172,61] 95% 169,81 172,61
(95% CI = X ± t ⋅ SE ) —> (tabel aflezen) t bij df = 55 (56 - 1)
2. Fill in the missing numbers. Round to two digits.
Give the 95% reference interval for the women from the south and give your interpretation of
it: […… ; ……]. About 95 of women from the south have a height between …… and …… cm.
Antwoord: 170,7654 ± 2,017 ⋅ 5,44 = [159.79; 181,74] 159,79 181,74
±
(95% reference interval = X t ⋅ std. deviation) —> t bij df = 43 (44 - 1)
3. Fill in the missing numbers.
Are the variances of the heights of the two groups equal yes/no ……
Antwoord: Yes (ja)
4. Fill in the missing numbers.
For testing the null hypothesis of equal means, which t-test would you use, the one with equal
variances assumed or the one without assuming equal variances: with/without …… equal
variances assumed?
Antwoord: With (met)
5. Fill in the missing numbers. Round to two digits
Test the null hypothesis μ north =μ south. Formulate your conclusion.
The difference between the means equals (rounded): …… cm. This leads to a t-value: …… .
The two sided P-value for this t-value in a t-distribution with 98 degrees of freedom (df) equals
…… . Since this P-value is larger/smaller …… than 0.05, we will reject/not reject …… the null
hypothesis of equal means. We do/do not …… have enough proof to state that the mean
heights of women from north and south differ.
Antwoord: 0,45 0,42 0,68 larger not reject do not
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