How are the numbers for video views (25%, 50%, 75%, 100%) calculated?

Regarding Yahoo, we use the figures obtained directly from the platform,
and for Google, we calculate them as follows:

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100% View Count = Impressions × 100% View Playback Ratio
100% View Rate = 100% View Count / Impressions
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※Since it is not possible to obtain a 100% view count from Google,
  depending on the period, there may be slight discrepancies between the figures
  in the platform management screen's view count × playback ratio.