Brush up M&T - studieboek - Design of experiment

13 important questions on Brush up M&T - studieboek - Design of experiment

What is the benefit of randomization?

Confounders independent of treatment (equally represented in both groups). So, bias due to confounding is eliminated
--> does not eliminate noise!

How can noise be reduced?

- randomization
- measure individuals twice (before and after) --> paired t-test
- controlling conditions
- precise measurements
- matching similar samples (blocking, cross-over etc)

When is a paired t-test used?

When individuals are measured twice (the results are paired to each other)
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What is a cross-over design?

Each animal receives treatment and non-treamtent
--> variation between animals is eliminated
--> not always possible
--> two groups, A, then B and B, then A

--> carry over can occur (therefor, include wash out period)
--> season effects can occur (timepoint is relevant)

What is a randomized block design?

- one variable is blocked (gender, age ...)
- used if confounders are observed
- two groups are made based on the blocked variable (female/male) and within the groups there will be randomization)
- blocking should also be used in analysis

What are factorial designs?

- investigate more treamtents at the same time
- naive: subjects exposed to all combinations
--> expert should be consulted

Why don't we maximize sample size always?

- cost time, money, labor etc. (economy
- animal suffering or death (ethics)
- very small differences not interesting (relevance)

What are the standard values tgaken for alpha and ß?

Alpha = 0.05
ß = 0.1 or 0.2

Power: pi = 1-ß (0.9vs. 0.8)

How can errors be controlled?

- pick alpha before the experiment and compare the p-value to this alpha
- every statistical test controls alpha correctly
- how is ß controlled?
  • ß is relevant only when there is a difference in the means
  • the larger the sample size, the smaller ß
- alpha is determined and ß is controlled by sample size (assumption)

What is the formula to calculate the sample size for a t-test?

Z1-alpha/2 = z1-0.05/2 = 1.96
Z1-ß = z1-0.2 or z1-0.1 = 0.84 or 1.28

Delta = difference between groups
n = sample size

What is the equation for the sample size for a paired t-test?

1-p is different, p = intraclass correlation between 0 and 1. Common choice = p=0.05

What is the equation for the sample size for a x2 test?

Pi1 = event frequency in group 1
pi2 = event frequency in group 2
pi mean = (pi1+pi2) / 2

What is the difference between a type I and type II error?

Type 1 = set
type 2 = error controlled by sample size

Errors are alpha and ß

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