3.1 Item information by ability
In order to illustrate the use of the item information by ability in test construction, we select two sets of 10 items for a potential \(D\)-score instrument. In Table ?? the two sets of example items are described. Both sets have items that appear to assess a wide range of developmental aspects.
| item | label | tau | item | label | tau |
|---|---|---|---|---|---|
| ddigmd057 | Lifts head to 45 degrees on prone position | 20.0 | iyosec001 | Does your child stop crying or calm down when you come to the room after being out of sight, or when you pick him or her up? | 30.2 |
| mdtgmd002 | Prone (on tummy), can lift head up to 90 degrees | 25.9 | denfmd007 | Reaches | 32.4 |
| iyosec001 | Does your child stop crying or calm down when you come to the room after being out of sight, or when you pick him or her up? | 30.2 | iyolgc004 | Does your child make single sounds like “buh” or “duh” or “muh?” | 33.0 |
| by1mdd054 | picks up cube (check hand preference) | 35.2 | dmcgmd003 | Sits supported | 33.6 |
| mdtgmd010 | Sits by self well | 40.0 | griehd011 | Hands explore table surface | 34.1 |
| gricgd022 | Finds toy under cup | 45.3 | by1mdd054 | picks up cube (check hand preference) | 35.2 |
| sgrred035 | Uses spoon | 50.5 | by3cgd051 | Blue Board Series: 1 piece | 55.1 |
| by3cgd051 | Blue Board Series: 1 piece | 55.1 | by3cgd059 | Attends to story | 60.0 |
| by3cgd059 | Attends to story | 60.0 | cromoc033 | Can the child unscrew the lid from a bottle or jar? | 64.2 |
| mulcgd200 | Matches by shape | 65.0 | mulcgd200 | Matches by shape | 65.0 |
The information for the first item set is presented in Figure 3.1. This set is very nicely distributed over the ability range. So this set of items provides information over the entire range of \(D\)-scores.
Figure 3.2 shows that the second set of items is less well distributed over the \(D\)-score scale. Six items provide information between 30 and 35 \(D\), there are no items between 40 and 50 \(D\) and items cromoc033 and mulcgd200 appear to inform a similar range (around 65\(D\)).
Figure 3.1: Item information by ability for a set of 10 items nicely distributed over the \(D\)-score scale.
Figure 3.2: Item information by ability for a set of 10 items not evenly distributed over the \(D\)-score scale.