Explanation

Most theories of memory assume that it consists of a short term and a long term store (e.g. Glietman, Fridlund and Reisberg, 2004). This experiment will investigate the effect of recall interval on short term (working) memory. Research has shown that working memory has a very limited capacity of around 7 items (Miller, 1956), and that information stored in working memory decays rapidly unless the information is rehearsed or encoded and transferred to long term memory. For example, you leave the house with a list of items on your shopping list and try to rehearse them in your mind (working memory). But on the way to the shops you have to post a letter, go to the bank, and then bump into someone you know and start chatting. These distractions will have prevented rehearsal of the items on your list, so by the time you reach the shop,you will have forgotten some of them. Would you have remembered more items if you hadn`t been distracted? In order to investigate the capacity of working memory when rehearsal is prevented, the following experiment will present subjects with 3 consonants and ask them to recall the stimuli at 2 different intervals. In condition 1 subjects will be asked to recall the stimuli after a 100 millisecond interval. In condition 2, subjects will be asked to recall the stimuli after a 1200 millisecond interval. In both conditions, subjects will have to complete subtraction problems during the recall interval. This distractor task will prevent subjects rehearsing the stimuli and allow us to investigate how working memory is affected by the length of time (recall interval) between presentation of the stimulus and subsequent recall. We would expect the items to decay from working memory as recall interval increases. This experiment is similar to a classic experiment by Peterson and Peterson (1959).

We will use the vocabulary of statistics, which can be confusing if you haven't seen it before, so here is an introduction to the study you will be working through and the words that are used to describe it.

The Data
Statistics are designed to help us understand things we observe in the world around us. To use statistics, we have to measure things in the real world and so produce data. Data can be expressed as words or numbers, and are plural - so you say "Here are my data."

So that we know which aspects of the data we are talking about, we use the following words:

• To generate data we take measurements or make observations of specific qualities of things;
• The things we are measuring are called the experimental units of the study. They might be referred to as 'people' or 'soil samples', whatever is being measured, but in this study, they are referred to as subjects;
• The qualities that we measure, or observe, are called variables. So if you measured a piece of string, the string would be the experimental unit and 'length' would be the variable;
• All variables take a range of values - the variable 'length' might take the values 3 or 10.5, for example. Generally, one measurement of a variable from a single experimental unit will produce a single value. If we say "Length = 5" then 'length' is the variable and '5' is the value.

Your study measured one variable: Items correctly Recalled, in two samples. Items correctly Recalled is measured in Items.

The Study
Experiments often compare one set of measurements with another. The two sets of measurements could differ because they were each taken from different groups of experimental units, or because they were taken from the same experimental units under two different conditions. Each set of measurements is called a sample. Your study splits the subjects into two samples:

• The 100 msecs sample
• The 1200msecs sample

Such studies always have two variables, each with its own role to play. They are:

• The independent variable discriminates between the two samples. Your independent variable is recall interval and it can take one of two values: 100 msecs or 1200msecs.
• The dependent variable is whatever you are measuring in each sample. In the case of this study, the dependent variable is items correctly recalled, so you expect items correctly recalled to differ depending on recall interval (whether it is 100 msecs or 1200msecs).

Such studies have an idea they wish to test. The idea is called the hypothesis. Each study actually has two versions of the hypothesis - one that says there is a difference between the two samples (the experimental hypothesis) and one that says that there is no difference (this is called the null hypothesis).

• The experimental hypothesis in this study is the number of items correctly recalled will decrease when the recall interval is increased.
• The null hypothesis in this study is the number of items correctly recalled will not be affected by recall interval.

Symbols
The list below shows you the symbols used in this tutorial to represent certain statistical measures. If the words are unfamiliar to you just now, don't worry this tutorial will make everything clear. You can refer back to this page at any point if you need to look up a word or a symbol.

• x = The sample mean - note the bar over the X. You can say 'the mean of X' or just 'X bar' when reading this.
• μ = The population mean (pronounced mew)
• S² = The sample variance (say S squared)
• σ = The population variance (pronounced sigma)
• S = The sample standard deviation
• σ² = The population standard deviation (sigma squared)

Exploration

The table below shows items correctly recalled for the both the 100 msecs and the 1200msecs samples.

Items correctly Recalled (Items)
100 msecs	1200msecs
10	2
11	5
12	3
12	3
13	8
13	1
13	4
13	2
14	4
14	5
14	2
14	0
14	4
15	3
15	3
15	0
15	5
16	1
17	2
18	3

Application

We are measuring items correctly recalled in your data. Which of the words in the box to the right best describes the role of items correctly recalled?	Choose..A valueA variableA sample
Recall Interval can be either 100 msecs or 1200msecs. What do these two words refer to?	Choose..VariablesSamplesHypotheses
Which of these is your null hypothesis?	Choose..The number of items correctly recalled will not be affected by recall intervalThe number of items correctly recalled will decrease when the recall interval is increased
Which variable is the independent variable?	Choose..Recall Interval Items correctly Recalled
Which variable is the dependent variable?	Choose..Recall Interval Items correctly Recalled
Imagine that you measured items correctly recalled from a subject in the 100 msecs sample and got a value of 11 Items.
Which of these words best describes items correctly recalled?	Choose..A variableA valueA sample
Which of these words best describes 100 msecs?	Choose..A variableA value and a sample
Which of these words best describes 11?	Choose..A variableA valueA sample