---
title: "assigning-participants"
aliases: 
tags: 
- info203
---


# Methods
## Between Subjects
The participants are split into equal groups, each which reviews one of the alternatives
## Within Subjects
The participants are split into two grops, one group review the first alternative first, the other review the seconds alternative first. 
## Latin Square
![](https://i.imgur.com/ZORPvqk.png)

order of each group is different
# Counterbalanced assignments
e.g., typing speed might affect interface usage
you can use a pretest to assign participants to that typing speed is roughly balanced

many techniques: key is to have equal chance of each participant in each group

## offline counterbalancing
pre-testing then forming matches pairs, which are split between groups]

## online counterbalancing
- of ther is no pre-test pick a threshold that is likely to be about the middle
- as they come in

- dont need to ensure even nubmer of high and low typers.
- do need to ensure the same number of high/low typers in a and b
## dangers
### regression:
- find heady coins
- first flip them all (pre-test)
- if they land heads more than half, call them heady
- "feed them a snack"
- does snacking increase the natural tendency of coins

![coin flip example](https://i.imgur.com/Y2CWCRV.png)

both regress towards the mean

### how to avoid
if the pretest is used to counterbalance, and assignment is random, then the error goes away


# should every participant use every alternative
three major srategies

- **Within** 
	- everyone tries all options
	- good when not fussed about learning/practise/exposure isssues
- **between participants**
	- each person tries only only of the options
	- requires more people, and more attention to fair assignment
	- has the benefit that each participant is uncorrupted
	- most common forthings like web studies
- **Counterbalancing**
	- 

# hawthorne effect
results are a result of the act of exmperimenting itself not as a resyult of the manupulations of the experiment

can be avoided with random assignment

# vaccum cleaner example
- manipulation
	- vacuum type
- measures
	- speed
	- cleanliness

- between subjects design: assign half the participants to each type.
	- worried about individual differences
- within subjects design: everyone uses both interfaces:
	- worried about ordering effects

- half try one first, the other try the other first (counterbalancing)
	- each of the tasks should be difference e.g., clean differnt buildings/rooms

individual differnces: go based of intuition of if it will make a difference.
Random assignment is importantg