init research

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+[//]: # (title: mean)
+
+<!---IMPORT org.jetbrains.kotlinx.dataframe.samples.api.Analyze-->
+
+Computes the [mean (average)](https://en.wikipedia.org/wiki/Arithmetic_mean) of values.
+
+`null` values are ignored.
+
+All primitive numeric types are supported: `Byte`, `Short`, `Int`, `Long`, `Float`, and `Double`.
+
+`mean` also supports the "mixed" `Number` type, as long as the column consists only of the aforementioned
+primitive numbers.
+The numbers are automatically converted to a [common type](numberUnification.md) for the operation.
+
+The return type is always `Double`; `Double.NaN` for empty columns.
+
+All operations on `Double`/`Float`/`Number` have the `skipNaN` option, which is
+set to `false` by default. This means that if a `NaN` is present in the input, it will be propagated to the result.
+When it's set to `true`, `NaN` values are ignored.
+
+<!---FUN meanModes-->
+
+```kotlin
+df.mean() // mean of values per every numeric column
+df.mean { age and weight } // mean of all values in `age` and `weight`
+df.meanFor(skipNaN = true) { age and weight } // mean of values per `age` and `weight` separately, skips NaN
+df.meanOf { (weight ?: 0) / age } // median of expression evaluated for every row
+```
+
+<!---END-->
+
+<!---FUN meanAggregations-->
+
+```kotlin
+df.mean()
+df.age.mean()
+df.groupBy { city }.mean()
+df.pivot { city }.mean()
+df.pivot { city }.groupBy { name.lastName }.mean()
+```
+
+<!---END-->
+
+See [statistics](summaryStatistics.md#groupby-statistics) for details on complex data aggregations.
+
+See [column selectors](ColumnSelectors.md) for how to select the columns for this operation.
+
+### Type Conversion
+
+The following automatic type conversions are performed for the `mean` operation:
+
+| Conversion                                                                 | Result for Empty Input |
+|----------------------------------------------------------------------------|------------------------|
+| Int -> Double                                                              | Double.NaN             |
+| Byte -> Double                                                             | Double.NaN             |
+| Short -> Double                                                            | Double.NaN             |
+| Long -> Double                                                             | Double.NaN             |
+| Double -> Double                                                           | Double.NaN             |
+| Float -> Double                                                            | Double.NaN             |
+| Number -> Conversion([Common number type](numberUnification.md)) -> Double | Double.NaN             |
+| Nothing -> Double                                                          | Double.NaN             |