Experimental Design Help

[mylertears]

I hope I am posting in the correct forum. A colleague and I gave a problem on a test recently that we're debating over. I hope that someone can help. It is as follows:

Which of the following statements if true about block design?

i. Treatments are assigned haphazardly to experimental units
ii. It controls for the effects of confounding variables
iii. Experimental units are examined before receiving treatments

(a) I only
(b) I and III only
(c) I and II only
(d) II only
(e) III only

I understand I is false and II is true, but we're debating over III. Since it is blocking I believe the experimental units would need to be 'examined' beforehand so that we know how to block them (knowing some characteristic about them seems to qualify as an examination to an extent). He thinks not. Clearly II and III is not an option, but I'm having a hard time picking (d) since it says II ONLY. Am I incorrect in my thinking?

 

[ondansetron]

What do they mean by 'haphazardly'?

 

[Miner]

Blocking is used to minimize the impact of a known or suspected source of variation that is not of interest in the experiment. All levels of the treatment should be equally represented (balanced) within each block. Experimental units should be randomly assigned to these treatment levels.

 

[Buckeye]

I like to think of the situation where i have 2 treatment medications for dogs. Let's say to treat fleas. If I know the dog breed, I can block by breed. One of the ideas of a block is that it makes the EUs within a block more homogenous (and thus explains variability). The block is a restriction on randomization because we randomize treatments within blocks. So, we have to know the block before assigning treatments. Another example is to block by age and gender when evaluating a physical fitness test. I vote that you are right.

^ We can also have balanced incomplete block designs where the block is not big enough to fit all treatments.

 

[mylertears]

I terms of hapharzardly, I think they mean with no care and/or organization.

Buckeye, that was my train of thought. I'm not sure I can get away without 'examining' the units before blocking. Maybe it's not an appropriate word to use for this question if they don't intend for it to be a part of the correct answer.

Thanks for the replies, any other insight would help if someone feels differently.