diff --git a/src/PhpSpreadsheet/Calculation/Statistical.php b/src/PhpSpreadsheet/Calculation/Statistical.php
index 3727cc12..eeee84fe 100644
--- a/src/PhpSpreadsheet/Calculation/Statistical.php
+++ b/src/PhpSpreadsheet/Calculation/Statistical.php
@@ -596,11 +596,7 @@ class Statistical
             }
             // Is it a numeric value?
             if ((is_numeric($arg)) && (!is_string($arg))) {
-                if ($returnValue === null) {
-                    $returnValue = $arg;
-                } else {
-                    $returnValue += $arg;
-                }
+                $returnValue += $arg;
                 ++$aCount;
             }
         }
@@ -643,11 +639,7 @@ class Statistical
                     } elseif (is_string($arg)) {
                         $arg = 0;
                     }
-                    if ($returnValue === null) {
-                        $returnValue = $arg;
-                    } else {
-                        $returnValue += $arg;
-                    }
+                    $returnValue += $arg;
                     ++$aCount;
                 }
             }
@@ -821,7 +813,7 @@ class Statistical
      * @param float $probability Probability of success on each trial
      * @param bool $cumulative
      *
-     * @return float
+     * @return float|string
      *
      * @todo    Cumulative distribution function
      */
@@ -863,7 +855,7 @@ class Statistical
      * @param float $value Value for the function
      * @param float $degrees degrees of freedom
      *
-     * @return float
+     * @return float|string
      */
     public static function CHIDIST($value, $degrees)
     {
@@ -896,7 +888,7 @@ class Statistical
      * @param float $probability Probability for the function
      * @param float $degrees degrees of freedom
      *
-     * @return float
+     * @return float|string
      */
     public static function CHIINV($probability, $degrees)
     {
@@ -955,7 +947,7 @@ class Statistical
      * @param float $stdDev Standard Deviation
      * @param float $size
      *
-     * @return float
+     * @return float|string
      */
     public static function CONFIDENCE($alpha, $stdDev, $size)
     {
@@ -985,7 +977,7 @@ class Statistical
      * @param mixed $yValues array of mixed Data Series Y
      * @param null|mixed $xValues array of mixed Data Series X
      *
-     * @return float
+     * @return float|string
      */
     public static function CORREL($yValues, $xValues = null)
     {
@@ -1513,7 +1505,7 @@ class Statistical
      * @param mixed $yValues array of mixed Data Series Y
      * @param mixed $xValues of mixed Data Series X
      *
-     * @return float
+     * @return bool|float|string
      */
     public static function FORECAST($xValue, $yValues, $xValues)
     {
@@ -1692,7 +1684,7 @@ class Statistical
     /**
      * GROWTH.
      *
-     * Returns values along a predicted emponential Trend
+     * Returns values along a predicted exponential Trend
      *
      * @param mixed[] $yValues Data Series Y
      * @param mixed[] $xValues Data Series X
diff --git a/tests/PhpSpreadsheetTests/Calculation/StatisticalTest.php b/tests/PhpSpreadsheetTests/Calculation/StatisticalTest.php
index 92fa9b78..97238913 100644
--- a/tests/PhpSpreadsheetTests/Calculation/StatisticalTest.php
+++ b/tests/PhpSpreadsheetTests/Calculation/StatisticalTest.php
@@ -109,6 +109,86 @@ class StatisticalTest extends TestCase
         return require 'data/Calculation/Statistical/BETAINV.php';
     }
 
+    /**
+     * @dataProvider providerBINOMDIST
+     *
+     * @param mixed $expectedResult
+     */
+    public function testBINOMDIST($expectedResult, ...$args)
+    {
+        $result = Statistical::BINOMDIST(...$args);
+        self::assertEquals($expectedResult, $result, '', 1E-12);
+    }
+
+    public function providerBINOMDIST()
+    {
+        return require 'data/Calculation/Statistical/BINOMDIST.php';
+    }
+
+    /**
+     * @dataProvider providerCHIDIST
+     *
+     * @param mixed $expectedResult
+     */
+    public function testCHIDIST($expectedResult, ...$args)
+    {
+        $result = Statistical::CHIDIST(...$args);
+        self::assertEquals($expectedResult, $result, '', 1E-12);
+    }
+
+    public function providerCHIDIST()
+    {
+        return require 'data/Calculation/Statistical/CHIDIST.php';
+    }
+
+    /**
+     * @dataProvider providerCHIINV
+     *
+     * @param mixed $expectedResult
+     */
+    public function testCHIINV($expectedResult, ...$args)
+    {
+        $result = Statistical::CHIINV(...$args);
+        self::assertEquals($expectedResult, $result, '', 1E-12);
+    }
+
+    public function providerCHIINV()
+    {
+        return require 'data/Calculation/Statistical/CHIINV.php';
+    }
+
+    /**
+     * @dataProvider providerCONFIDENCE
+     *
+     * @param mixed $expectedResult
+     */
+    public function testCONFIDENCE($expectedResult, ...$args)
+    {
+        $result = Statistical::CONFIDENCE(...$args);
+        self::assertEquals($expectedResult, $result, '', 1E-12);
+    }
+
+    public function providerCONFIDENCE()
+    {
+        return require 'data/Calculation/Statistical/CONFIDENCE.php';
+    }
+
+    /**
+     * @dataProvider providerCORREL
+     *
+     * @param mixed $expectedResult
+     */
+    public function testCORREL($expectedResult, array $xargs, array $yargs)
+    {
+        $result = Statistical::CORREL($xargs, $yargs);
+        self::assertEquals($expectedResult, $result, '', 1E-12);
+    }
+
+    public function providerCORREL()
+    {
+        return require 'data/Calculation/Statistical/CORREL.php';
+    }
+
     /**
      * @dataProvider providerCOUNTIF
      *
@@ -141,6 +221,38 @@ class StatisticalTest extends TestCase
         return require 'data/Calculation/Statistical/COUNTIFS.php';
     }
 
+    /**
+     * @dataProvider providerCOVAR
+     *
+     * @param mixed $expectedResult
+     */
+    public function testCOVAR($expectedResult, ...$args)
+    {
+        $result = Statistical::COVAR(...$args);
+        self::assertEquals($expectedResult, $result, '', 1E-12);
+    }
+
+    public function providerCOVAR()
+    {
+        return require 'data/Calculation/Statistical/COVAR.php';
+    }
+
+    /**
+     * @dataProvider providerFORECAST
+     *
+     * @param mixed $expectedResult
+     */
+    public function testFORECAST($expectedResult, ...$args)
+    {
+        $result = Statistical::FORECAST(...$args);
+        self::assertEquals($expectedResult, $result, '', 1E-12);
+    }
+
+    public function providerFORECAST()
+    {
+        return require 'data/Calculation/Statistical/FORECAST.php';
+    }
+
     /**
      * @dataProvider providerMAXIFS
      *
diff --git a/tests/data/Calculation/Statistical/AVERAGEIF.php b/tests/data/Calculation/Statistical/AVERAGEIF.php
index 94b860ff..bd976471 100644
--- a/tests/data/Calculation/Statistical/AVERAGEIF.php
+++ b/tests/data/Calculation/Statistical/AVERAGEIF.php
@@ -45,4 +45,9 @@ return [
         [2012, 2012, 2013, 2011, 2011, 2010],
         "<2013",
     ],
+    [
+        14000,
+        [7000, 14000, 'Hello World', 21000, 28000],
+        "<23000",
+    ],
 ];
diff --git a/tests/data/Calculation/Statistical/BETADIST.php b/tests/data/Calculation/Statistical/BETADIST.php
index 7e32ba57..5e965736 100644
--- a/tests/data/Calculation/Statistical/BETADIST.php
+++ b/tests/data/Calculation/Statistical/BETADIST.php
@@ -9,4 +9,8 @@ return [
         0.598190307617,
         7.5, 8, 9, 5, 10,
     ],
+    [
+        0.685470581054,
+        2, 8, 10, 1, 3,
+    ],
 ];
diff --git a/tests/data/Calculation/Statistical/BETAINV.php b/tests/data/Calculation/Statistical/BETAINV.php
index ffd5f31e..7d0657be 100644
--- a/tests/data/Calculation/Statistical/BETAINV.php
+++ b/tests/data/Calculation/Statistical/BETAINV.php
@@ -9,4 +9,8 @@ return [
         7.761240188783,
         0.75, 8, 9, 5, 10,
     ],
+    [
+        2.0,
+        0.685470581055, 8, 10, 1, 3,
+    ],
 ];
diff --git a/tests/data/Calculation/Statistical/BINOMDIST.php b/tests/data/Calculation/Statistical/BINOMDIST.php
new file mode 100644
index 00000000..40770781
--- /dev/null
+++ b/tests/data/Calculation/Statistical/BINOMDIST.php
@@ -0,0 +1,20 @@
+<?php
+
+return [
+    [
+        0.706399436132,
+        3, 8, 0.35, true,
+    ],
+    [
+        0.538748204875,
+        50, 125, 0.4, true,
+    ],
+    [
+        0.278585779062,
+        3, 8, 0.35, false,
+    ],
+    [
+        0.205078125,
+        6, 10, 0.5, false,
+    ],
+];
diff --git a/tests/data/Calculation/Statistical/CHIDIST.php b/tests/data/Calculation/Statistical/CHIDIST.php
new file mode 100644
index 00000000..26940581
--- /dev/null
+++ b/tests/data/Calculation/Statistical/CHIDIST.php
@@ -0,0 +1,16 @@
+<?php
+
+return [
+    [
+        0.964294972685,
+        3, 9,
+    ],
+    [
+        0.4837673815536,
+        7.5, 8,
+    ],
+    [
+        0.050000589092,
+        18.307, 10,
+    ],
+];
diff --git a/tests/data/Calculation/Statistical/CHIINV.php b/tests/data/Calculation/Statistical/CHIINV.php
new file mode 100644
index 00000000..44e7a2ba
--- /dev/null
+++ b/tests/data/Calculation/Statistical/CHIINV.php
@@ -0,0 +1,16 @@
+<?php
+
+return [
+    [
+        8.383430828608,
+        0.3, 7,
+    ],
+    [
+        6.737200771955,
+        0.75, 10,
+    ],
+    [
+        18.30697345702,
+        0.050001, 10,
+    ],
+];
diff --git a/tests/data/Calculation/Statistical/CONFIDENCE.php b/tests/data/Calculation/Statistical/CONFIDENCE.php
new file mode 100644
index 00000000..361b1360
--- /dev/null
+++ b/tests/data/Calculation/Statistical/CONFIDENCE.php
@@ -0,0 +1,8 @@
+<?php
+
+return [
+    [
+        0.692951912734,
+        0.05, 2.5, 50,
+    ],
+];
diff --git a/tests/data/Calculation/Statistical/CORREL.php b/tests/data/Calculation/Statistical/CORREL.php
new file mode 100644
index 00000000..dcdbef3b
--- /dev/null
+++ b/tests/data/Calculation/Statistical/CORREL.php
@@ -0,0 +1,9 @@
+<?php
+
+return [
+    [
+        0.997054485502,
+        [3, 2, 4, 5, 6],
+        [9, 7, 12, 15, 17],
+    ],
+];
diff --git a/tests/data/Calculation/Statistical/COVAR.php b/tests/data/Calculation/Statistical/COVAR.php
new file mode 100644
index 00000000..766dad86
--- /dev/null
+++ b/tests/data/Calculation/Statistical/COVAR.php
@@ -0,0 +1,14 @@
+<?php
+
+return [
+    [
+        5.2,
+        [3, 2, 4, 5, 6],
+        [9, 7, 12, 15, 17],
+    ],
+    [
+        1.25,
+        [[1, 2], [3, 4]],
+        [[5, 6], [7, 8]],
+    ]
+];
diff --git a/tests/data/Calculation/Statistical/FORECAST.php b/tests/data/Calculation/Statistical/FORECAST.php
new file mode 100644
index 00000000..c6abbb65
--- /dev/null
+++ b/tests/data/Calculation/Statistical/FORECAST.php
@@ -0,0 +1,40 @@
+<?php
+
+return [
+    [
+        10.607253086419,
+        30,
+        [6, 7, 9, 15, 21],
+        [20, 28, 31, 38, 40],
+    ],
+    [
+        11.8937852010975,
+        5,
+        [10.5, 7.2, 200, 5.4, 8.1],
+        [-3, 4, 120, 2, 7.5],
+    ],
+    [
+        20.032698660553,
+        10,
+        [10.5, 7.2, 200, 5.4, 8.1],
+        [-3, 4, 120, 2, 7.5],
+    ],
+    [
+        5.0,
+        8,
+        [1, 2, 3],
+        [4, 5, 6],
+    ],
+    [
+        10.657142857143,
+        7,
+        [5.8, -1],
+        [2, -5],
+    ],
+    [
+        -5.0,
+        50,
+        [-1, -2, -3, -4],
+        [10, 20, 30, 40],
+    ],
+];