[GitHub] [madlib] hpandeycodeit commented on issue #432: MADLIB-1351 : Added stopping criteria on perplexity to LDA

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hpandeycodeit commented on issue #432: MADLIB-1351 : Added stopping criteria on perplexity to LDA
URL: https://github.com/apache/madlib/pull/432#issuecomment-548876785
 
 
   @fmcquillan99, 
   
   In `lda_predict ` although the model table remains the same, it randomly initializes the output table. That is why we are seeing the difference in the perplexity values from what is calculated in `lda_train` vs `get_perplexity()`
   
   However, if the same output table(generated by `lda_train`) is passed to the `get_perplexity()` function, the perplexity values match. For eg: 
   
   ```
   DROP TABLE IF EXISTS lda_model_perp, lda_output_data_perp;
   
   SELECT madlib.lda_train( 'documents_tf',          -- documents table in the form of term frequency
                            'lda_model_perp',        -- model table created by LDA training (not human readable)
                            'lda_output_data_perp',  -- readable output data table 
                            385,                     -- vocabulary size
                            5,                        -- number of topics
                            10,                      -- number of iterations
                            5,                       -- Dirichlet prior for the per-doc topic multinomial (alpha)
                            0.01,                    -- Dirichlet prior for the per-topic word multinomial (beta)
                            1,                       -- Evaluate perplexity every n iterations
                            .2                      -- Stopping perplexity tolerance
                          );
   
   ```
   
   Generates the following perplexity values with the last perplexity value **179.380131412**: 
   
   ```
   postgres=# select perplexity from lda_model_perp ;
                                                                     perplexity                                                                  
   ----------------------------------------------------------------------------------------------------------------------------------------------
    {196.940707618,193.245742228,191.155602156,185.314159394,182.901929923,187.283749958,186.944341124,185.508311039,185.72038473,179.380131412}
   (1 row)
   
   ```
   
   Now running the `get_perplexity()` on the above-generated output table  `lda_output_data_perp` produces the following perplexity:
   
   ```
   postgres=# SELECT madlib.lda_get_perplexity( 'lda_model_perp',
   postgres(#                                   'lda_output_data_perp'
   postgres(#                                 );
    lda_get_perplexity 
   --------------------
      179.380131412469
   ```
   
   which matches the last perplexity value calculated by `lda_train`
   
   Thanks! 
   

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