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*Dear Professor Mean

- I have the following data in a two by two table:*

::: {align=“left”}

```
**D+** **D-** **Total**
**F+** **34** **23** **57**
**F-** **139** **119** **258**
```

**Total** **173** **142** **315**

:::

*When I try to enter this data into SPSS

- I can’t get it to compute risk ratios and confidence intervals. What am I doing wrong? – Jinxed Jason*

Dear Jinxed,

You have values ranging from F- to D+? I hope this isn’t data on the grades you received in college.

Actually these data are from a paper: Sands et al (1999). F+ represents presence of a risk factor (in this case

- previous miscarriage) and F- represents absence of that risk factor. D+ represents presence of a defect (ventricular septal defect or VSD) and D- represents absence of that defect.

::: {align=“left”}

```
**Risk **Group** **Number/Total **Odds Ratio
Factor** (Percent)** (95%CI)**
```

**Miscarriage** **VSD 34/173 (20%) 1.3 (0.7,2.3)
Control 23/142 (16%)**

```
**Female** **VSD **84/173 (49%) **2.1 (1.3,3.2)**
Control** 60/142 (42%)**
```

**Low parity** **VSD 76/173 (44%) 1.1 (0.7,1.8)
Control 58/142 (41%)**

```
**Smoking** **VSD **41/173 (24%) **0.8 (0.5,1.3)**
Control** 39/139 (28%)**
**Alcohol** **VSD **18/173 (10%) **0.7 (0.4,1.5)**
Control** 20/139 (14%)**
```

:::

Notice that we have to do a bit of arithmetic to get all the values. **If 34 out of 173 VSD cases had a previous miscarriage

- then 139=173-34 did not. If 23 out of 142 controls had previous miscarriage as a risk factor
- then 119 did not.**

For data like this

- you have to re-arrange things and then apply weights. The following discussion talks about SPSS
- but the general method works for most other statistical software.

To re-arrange the data

- you need to specify three variables: F
- D
- and
COUNT.
**F takes the value of 1 for F+ and 0 for F-. D takes the value of 1 for D+ and 0 for D-**. The 0-1 coding has some nice mathematical properties - but you could use 1 and 2 instead.
**For each combination of F and D we will record the sample size in COUNT**.

Here’s what your re-arranged data would look like

**Enter the data

- and tell SPSS that W represents a weighting variable**
- and you’re ready to rock and roll. You do this by
selecting
**Data | Weight Cases from the SPSS menu.**

Then select **Analyze | Descriptive Statistics | Crosstabs from the
SPSS menu** to create a two by two table.

Be sure to click on the **Statistics button** and select the **Risk
option box** to ask SPSS to compute the risk ratios.

I also usually find it useful to display the row percentages. To do this

- click on the
**Cells button**.

In the **Crosstabs: Cell Display dialog box**

- select the
**Row Percentages option box**.

Here’s what the first part of the output looks like.

Notice that the rows and columns are reversed in this table. There are several ways to change how the table is displayed

- but it is showing essentially the same information in any order.

Here is what the second part of the output looks like.

By the way

- if you tried to use the crosstabs procedure without weighting
- you would get exactly one observation in each cell. Pretty boring
- eh?

**Summary**

Jinxed Jason can’t figure out **how to enter data from a two by two
table into SPSS**. Professor Mean explains that **you need three
variables to represent a two by two table**. The first variable
indicates **the specific column** of your table and the second
variable indicates **the specific row** (or vice versa). The third
variable indicates **the count or frequency** for each intersection of
row and column. You do not include the row or column totals in your
data entry. You can then **select** **Analyze | Descriptive
Statistics | Crosstabs from the SPSS menu** **to analyze the data
from your two by two table**. You get additional analyses by selecting
the **Risk and/or Chi-square option boxes**.

**Further reading**

**Incidence and risk factors for ventricular septal defect in “low risk” neonates.**Sands AJ

- Casey F
- Craig B
- Dornan J
- Rogers J and Mulholland H. Arch Dis Child Fetal Neonatal Ed 1999:81(1);F61-F63. This paper is available on the web.

You can find an earlier version of this page on my original website.