# Sib-pair Command: twin

 Class Analysis and data manipulation command Name twin Arguments [ even | odd | (>|>=|<|<=|==|^= )]

Calculate, and test for equality of, MZ and DZ twin correlations or concordances. If a zygosity indicator is not specified, the indicator declared by the set twin command is used to divide siblings into MZ twins and non-MZ full siblings (DZ twins and "singletons"). If a zygosity indicator is specified, then this is used to define three groups: MZ twins (indicator expression TRUE or greater than zero), DZ twins (indicator FALSE or equal to zero), and nontwin siblings (indicator value is missing). The Z test for equality of the Pearson correlation coefficients is actually the likelihood ratio test [Brandner 1933].

Example:

```
>> twin ace zyg == 1

------------------------------------------------------------
Classical twin analysis of "ace"
------------------------------------------------------------

Zygosity Group  N Pairs        Mean    Std Dev     Correlation (95%CI)
--------------  ------- ------------  ---------  -------------------------
MZ twins           103     126.7573    48.6038    0.661 ( 0.536 --  0.757)
DZ twins           123     130.6057    51.5562    0.466 ( 0.315 --  0.594)

MZ Female           73     121.5068    48.2225    0.675 ( 0.527 --  0.783)
MZ Male             30     139.5333    47.5279    0.589 ( 0.527 --  0.783)
DZ Female           51     130.0784    48.9865    0.414 ( 0.157 --  0.619)
DZ Male             25     148.7200    49.2035    0.648 ( 0.340 --  0.830)
DZ Female-Male      47     113.4468,   48.2790,   0.423 ( 0.155 --  0.633)
129.6383    57.6136

Hypothesis      Z-statistic   P-value
--------------- -----------   -------
r(MZF) = r(MZM)        0.66    0.5099 .
r(DZF) = r(DZM)        1.35    0.1772 .
r(MZ) = r(DZ)          2.16    0.0153 +

Falconer style estimates of heritability

Heritability h2= 0.390
Domesticity  c2= 0.271

```

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