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POHow do I use Holt-Winters Seasonal Dampened Method to compute a two-month sales projection in PHP?
 

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  1. POHow do I use Holt-Winters Seasonal Dampened Method to compute a two-month sales projection in PHP?
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    <p>Holt-Winters is introduced here:</p> <p><a href="http://en.wikipedia.org/wiki/Holt-Winters" rel="nofollow">http://en.wikipedia.org/wiki/Holt-Winters</a></p> <p>The Seasonal Dampened version of it is discussed here (scroll down the page):</p> <p><a href="http://otexts.com/fpp/7/5/" rel="nofollow">http://otexts.com/fpp/7/5/</a></p> <p>In a nutshell, it basically looks at 3 things:</p> <ul> <li>long-term trend</li> <li>short-term trend</li> <li>seasonal trend</li> </ul> <p>It also doesn't average those together, because really what you need is weighted averaging, where seasonal and short-term are more significant than long-term trend, naturally, with financial data trends.</p> <p><em>Given $anYear1 and $anYear2, how do I apply the Holt-Winters Seasonal Dampened Method to forecast 2 more months past the end of $anYear2?</em> Assume $anYear1 is an array of 12 numbers. Assume $anYear2 is an array of a range of 0 to 12 numbers.</p> <p>So, I can fill it with random data like so:</p> <pre><code>&lt;?php $anYear1 = array(); $anYear2 = array(); $nStop = 10; // so we need 11 and 12 of the year for ($i = 1; $i &lt;= 12; $i++) { $anYear1[$i] = rand(200,500); if ($i &lt;= $nStop) { // give it a natural lift like real financial data $anYear2[$i] = rand(400,700); } } $nSeasonRange = 4; // 4 months in a business quarter Therefore, I want to create a function like so: function forecastHoltWinters($anYear1, $anYear2, $nSeasonRange = 4) { /////////////////// // DO MAGIC HERE // /////////////////// // an array with 2 numbers, indicating 2 months forward from end of $anYear2 return $anForecast; } $anForecast = forecastHoltWinters($anYear1, $anYear2, $nSeasonRange); echo "YEAR 1\n"; print_r($anYear1); echo "\n\nYEAR 2\n" print_r($anYear2); echo "\n\nTWO MONTHS FORECAST\n"; print_r($anForecast); </code></pre> <p>Note: I have found a Github example <a href="https://github.com/ianbarber/PHPIR/blob/master/holtwinters.php" rel="nofollow">here</a>, but it doesn't show how to do a projection. It is also discussed <a href="http://phpir.com/smoothing-with-holt-winter" rel="nofollow">here</a>.</p>
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    1. COKicking myself that I didn't take Calculus in college!
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