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            <h2><font color="#ffffff">Search Algorithms: BPC </font></h2>
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<font color="#000000"><b><br>
</b><b>Introduction </b></font>
<p>Build Pure Clusters (BPC) is one of the three algorithms in Tetrad
    designed to build <font color="#000000"><a
            href="../../definitions/measurement_structural_graph.html">pure
        measurement/structural models</a></font> (the others are the <font
            color="#000000"><a href="../../search/mimbuild.html">MIM Build algorithm</a></font>
    and the <font color="#000000"><a href="../../search/purify.html">Purify algorithm</a></font>).</p>
<p>The goal of Build Pure Clusters is to build a pure measurement model
    using observed variables from a data set. Observed variables are
    clustered into disjoint groups, each group representing
    indicators of a single hidden variable. Variables in one group are not
    indicators of the hidden variables associated with the other groupsl. Also, some variables given as input will not
    be
    used because they do not fit into a pure measurement model along with
    the chosen ones.</p>
<p>The Build Pure Clusters algorithm assumes that the population can be
    described as a measurement/structural model where observed variab<font
            color="#000000">les are linear indicators of the unknown latents.
        Notice that linearity among latents is not necessary (although it will
        be necessary for the <a href="../../search/mimbuild.html">MIM Build algorithm</a>)
        and latents do not need to be continuous. It is also assumed that the
        unknown population graph contains a pure subgraph where each latent has
        at least three indicators. This assumption is not testable is should be
        evaluated by the plausibility of the final model.</font></p>
<p><font color="#000000">The current implementation of the algorithm
    accepts only continuous data sets as input. For general information
    about model building algorithms, consult the <font color="#000000"><a
            href="../../search/../search.html">Search Algorithms</a></font> page.</font></p>
<p><font color="#000000"><a name="Introduction" id="Introduction"></a><br>
    Entering Build Pure Clusters parameters</font></p>
<p>For example, consider a model with this true graph: <br>
</p>
<blockquote>
    <p><font color="#000000"><img src="../../images/mimbuild1.png"
                                  width="769" height="470"></font></p>
</blockquote>
<p>If data is generated using this model and a search is constructed from the data, selecting BPC, the following
    parameters will be requested: </p>
<ul>
    <li><em>depErrorsAlpha value</em>: Build Pure Clusters uses
        statistical hypothesis tests in order to generate models automatically.
        The depErrorsAlpha value parameter represents the level by which such tests are
        used to accept or reject constraints that compose the final output. The
        default value is 0.05, but the user may want to experiment with
        different depErrorsAlpha values in order to test the sensitivity of her data
        within this algorithm.
    </li>
    <li><em>number of iterations</em>: Build Pure Clusters uses a
        randomized procedure in order to generate a model, since in general
        there are different pure measurement submodels of a given general
        measurement/structural model. This option allows the use to specify a
        given number of runs of the algorithm, where the outputs given for each
        run are combined together into s single model. This usually provides
        models that are more robust against statistical flunctuations and
        slight deviances from the assumptions.
    </li>
    <li><em>statistical test</em>: as stated before, automated
        model building is done by testing statistical hypothesis. Build Pure
        Clusters provides two basic statistical tests that can be used.
        Wishart's Tetrad ssumes that the given variables follow a multivariate
        normal distribution. Bollen's Tetrad test not make this assumption.
        However, it needs to compute a matrix of fourth moments, which can be
        time consuming. It is also less robust against sampling variability
        when compared to Wishart's test if the data actually follows a
        multivariate normal distribution..
    </li>
</ul>
<p><font color="#000000"><a name="Interpretation" id="Interpretation"></a><br>
    Interpreting the Output.
</font></p>
<p><font color="#000000">Upon executin the search, BPC returns a pure measurement
    model. Because of the internal randomization, outputs may vary from run
    to run, but one should not expect large differences (and this can be
    actually used to evaluate if the assumptions are reasonable for a given
    set of input variables). In our example, the outcome should be as
    follows if the sample is representative of the population:</font></p>
<blockquote>
    <p><font color="#000000"><img src="../../images/cluster2.png" width="536"
                                  height="264"></font></p>
</blockquote>
<p>Edges with circles at the endpoints are added only to distinguish
    latent variables from the indicators. BPC does not make
    any claims about the causal relationships among latent variables (this
    is the role of the <font color="#000000"><a href="../../search/mimbuild.html">MIM
        Build algorithm</a></font>). The labels given to the latent
    variables are arbitrary. As part of the analysis, a domain expert
    should evaluate if such latents have indeed a physical or abstract
    meaning, or if they should be discarded as meaningless. Such
    reification is domain dependent.</p>
<p>Note: If the output is not arranged helpfully, use the Fruchterman-Reingold layout in the Layout menu to arrange more
    readably. <br>
</p>
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