adding docs for gwr
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## Regression
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### Geographically weighted regression
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Can currently estimate Gaussian, Poisson, and logistic models (built on a GLM framework). GWR object prepares model input. Fit method performs estimation and returns a GWR Results object.
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#### Arguments
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| Name | Type | Description |
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|------|------|-------------|
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| subquery | text | SQL query that expose the data to be analyzed (e.g., `SELECT * FROM regression_inputs`). This query must have the geometry column name (see the optional `geom_col` for default), the id column name (see `id_col`), dependent and independent column names. |
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| dep_var | text | name of the dependent variable in the regression model |
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| ind_vars | text[] | Text array of independent used in the model to describe the dependent variable |
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| bw (optional) | numeric | bandwidth value consisting of either a distance or N nearest neighbors. Defaults to calculate an optimal bandwidth. |
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| fixed (optional) | boolean | True for distance based kernel function and False for adaptive (nearest neighbor) kernel function (default). Defaults to false. |
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| kernel | text | Type of kernel function used to weight observations. One of gaussian, bisquare (default), or exponential. |
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#### Returns
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| Column Name | Type | Description |
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|-------------|------|-------------|
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| coeffs | JSON | JSON object with parameter estimates for each of the dependent variables. The keys of the JSON object are the dependent variables, with values corresponding to the parameter estimate. |
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| stand_errs | JSON | Standard errors for each of the dependent variables. The keys of the JSON object are the dependent variables, with values corresponding to the respective standard errors. |
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| t_vals | JSON | T-values for each of the dependent variables. The keys of the JSON object are the dependent variable names, with values corresponding to the respective t-value. |
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| predicted | numeric | predicted value of y |
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| residuals | numeric | residuals of the response |
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| r_squared | numeric | R-squared for the parameter fit |
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| bandwidth | numeric | bandwidth value consisting of either a distance or N nearest neighbors |
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| rowid | int | row id of the original row |
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#### Example Usage
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```sql
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SELECT
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g.cartodb_id,
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g.the_geom,
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g.the_geom_webmercator,
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(gwr.coeffs->>'pctblack')::numeric as coeff_pctblack,
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(gwr.coeffs->>'pctrural')::numeric as coeff_pctrural,
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(gwr.coeffs->>'pcteld')::numeric as coeff_pcteld,
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(gwr.coeffs->>'pctpov')::numeric as coeff_pctpov,
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gwr.residuals
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FROM cdb_crankshaft.CDB_GWR('select * from g_utm'::text, 'pctbach'::text, Array['pctblack', 'pctrural', 'pcteld', 'pctpov']) As gwr
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JOIN g_utm as g
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on g.cartodb_id = gwr.rowid
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```
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Note: See [PostgreSQL syntax for parsing JSON objects](https://www.postgresql.org/docs/9.5/static/functions-json.html).
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