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    <title>Introduction to TETRAD</title>
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            <h2><font color="white">Graph Edge Types</font></h2>
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        <th nowrap>Edge Types</th>
        <th>Present Relationships</th>
        <th>Absent Relationships</th>
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        <td>A --&gt; B</td>
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            A is a cause of B.<br/>
            It may be a direct or indirect cause that may include other measured variables. Also, there may be an
            unmeasured confounder of A and B.
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        <td>B is not a cause of A</td>
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        <td>A &lt;-&gt; B</td>
        <td>There is an unmeasured confounder (call it L) of A and B. There may be measured variables along the causal
            pathway from L to A or from L to B.
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            A is not a cause of B.<br/>
            B is not a cause of A.
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        <td>A o-&gt; B</td>
        <td>Either A is a cause of B (i.e, A --&gt; B) or there is an unmeasured confounder of A and B (i.e, A &lt;-&gt;
            B) or both.
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        <td>B is not a cause of A.</td>
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        <td>A o-o B</td>
        <td>
            Exactly one of the following holds:
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                <li>A is a cause of B</li>
                <li>B is a cause of A</li>
                <li>there is an unmeasured confounder of A and B</li>
                <li>both a and c</li>
                <li>both b and c</li>
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        <td colspan="3">If an edge is solid, that means there is no latent
            confounder (i.e., is visible). If dashed, there is possibly latent confounder.
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        <td colspan="3">If an edge is thickened that means it is
            definitely direct. Otherwise, it is possibly direct.
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