diff --git a/src/average.rs b/src/average.rs
index d6c8f63..d28dbbe 100644
--- a/src/average.rs
+++ b/src/average.rs
@@ -31,16 +31,21 @@ impl Average {
         Average { avg: 0., n: 0, v: 0. }
     }
 
-    /// Add a number to the sequence of which the average is calculated.
-    pub fn add(&mut self, x: f64) {
+    /// Add a sample to the sequence of which the average is calculated.
+    pub fn add(&mut self, sample: f64) {
         // This algorithm introduced by Welford in 1962 trades numerical
         // stability for a division inside the loop.
         //
         // See https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance.
         self.n += 1;
-        let delta = x - self.avg;
+        let delta = sample - self.avg;
         self.avg += delta / f64::approx_from(self.n).unwrap();
-        self.v += delta * (x - self.avg);
+        self.v += delta * (sample - self.avg);
+    }
+
+    /// Determine whether the sequence is empty.
+    pub fn is_empty(&self) -> bool {
+        self.n == 0
     }
 
     /// Return the mean of the sequence.
@@ -139,18 +144,19 @@ mod tests {
     use core::iter::Iterator;
 
     #[test]
-    fn average_trivial() {
+    fn trivial() {
         let mut a = Average::new();
         assert_eq!(a.len(), 0);
         a.add(1.0);
         assert_eq!(a.mean(), 1.0);
         assert_eq!(a.len(), 1);
         assert_eq!(a.sample_variance(), 0.0);
+        assert_eq!(a.population_variance(), 0.0);
         assert_eq!(a.error(), 0.0);
     }
 
     #[test]
-    fn average_simple() {
+    fn simple() {
         let a: Average = (1..6).map(f64::from).collect();
         assert_eq!(a.mean(), 3.0);
         assert_eq!(a.len(), 5);
@@ -159,7 +165,7 @@ mod tests {
     }
 
     #[test]
-    fn average_numerically_unstable() {
+    fn numerically_unstable() {
         // The naive algorithm fails for this example due to cancelation.
         let big = 1e9;
         let sample = &[big + 4., big + 7., big + 13., big + 16.];
@@ -168,7 +174,7 @@ mod tests {
     }
 
     #[test]
-    fn average_normal_distribution() {
+    fn normal_distribution() {
         use rand::distributions::{Normal, IndependentSample};
         let normal = Normal::new(2.0, 3.0);
         let mut a = Average::new();
diff --git a/src/lib.rs b/src/lib.rs
index f232988..879fea1 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -6,5 +6,7 @@ extern crate conv;
 
 #[macro_use] mod macros;
 mod average;
+mod weighted_average;
 
 pub use average::Average;
+pub use weighted_average::WeightedAverage;
diff --git a/src/weighted_average.rs b/src/weighted_average.rs
new file mode 100644
index 0000000..4411b28
--- /dev/null
+++ b/src/weighted_average.rs
@@ -0,0 +1,137 @@
+use core;
+
+/// Represent the weighted arithmetic mean and the weighted variance of a
+/// sequence of numbers.
+#[derive(Debug, Clone)]
+pub struct WeightedAverage {
+    /// Sum of the weights.
+    weight_sum: f64,
+    /// Sum of the squares of the weights.
+    weight_sum_sq: f64,
+    /// Average value.
+    avg: f64,
+    /// Intermediate sum of squares for calculating the variance.
+    v: f64,
+}
+
+impl WeightedAverage {
+    /// Create a new weighted average.
+    pub fn new() -> WeightedAverage {
+        WeightedAverage { weight_sum: 0., weight_sum_sq: 0., avg: 0., v: 0. }
+    }
+
+    /// Add a sample to the weighted sequence of which the average is calculated.
+    pub fn add(&mut self, sample: f64, weight: f64) {
+        // This algorithm was suggested by West in 1979.
+        //
+        // See
+        // https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance.
+        self.weight_sum += weight;
+        self.weight_sum_sq += weight*weight;
+        let prev_avg = self.avg;
+        self.avg = prev_avg + (weight / self.weight_sum) * (sample - prev_avg);
+        self.v += weight * (sample - prev_avg) * (sample - self.avg);
+    }
+
+    /// Determine whether the sequence is empty.
+    pub fn is_empty(&self) -> bool {
+        self.weight_sum_sq == 0.
+    }
+
+    /// Return the sum of the weights.
+    pub fn sum_weights(&self) -> f64 {
+        self.weight_sum
+    }
+
+    /// Return the sum of the squared weights.
+    pub fn sum_weights_sq(&self) -> f64 {
+        self.weight_sum_sq
+    }
+
+    /// Return the weighted mean of the sequence.
+    pub fn mean(&self) -> f64 {
+        self.avg
+    }
+
+    /// Calculate the population variance of the weighted sequence.
+    ///
+    /// This assumes that the sequence consists of the entire population and the
+    /// weights represent *frequency*.
+    pub fn population_variance(&self) -> f64 {
+        if self.is_empty() {
+            0.
+        } else {
+            self.v / self.weight_sum
+        }
+    }
+
+    /// Calculate the unbiased sample variance of the weighted sequence.
+    ///
+    /// This assumes that the sequence consists of samples of a larger
+    /// population and the weights represent *frequency*.
+    pub fn sample_variance(&self) -> f64 {
+        if self.is_empty() {
+            0.
+        } else {
+            self.v / (self.weight_sum - 1.0)
+        }
+    }
+
+    /// Calculate the reliability variance of the weighted sequence.
+    ///
+    /// This assumes weights represent *reliability*.
+    pub fn reliability_variance(&self) -> f64 {
+        if self.is_empty() {
+            0.
+        } else {
+            self.v / (self.weight_sum - self.weight_sum_sq / self.weight_sum)
+        }
+    }
+}
+
+impl core::default::Default for WeightedAverage {
+    fn default() -> WeightedAverage {
+        WeightedAverage::new()
+    }
+}
+
+impl core::iter::FromIterator<(f64, f64)> for WeightedAverage {
+    fn from_iter<T>(iter: T) -> WeightedAverage
+        where T: IntoIterator<Item=(f64, f64)>
+    {
+        let mut a = WeightedAverage::new();
+        for (i, w) in iter {
+            a.add(i, w);
+        }
+        a
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use core::iter::Iterator;
+
+    #[test]
+    fn trivial() {
+        let mut a = WeightedAverage::new();
+        assert_eq!(a.sum_weights(), 0.);
+        assert_eq!(a.sum_weights_sq(), 0.);
+        a.add(1.0, 1.0);
+        assert_eq!(a.mean(), 1.0);
+        assert_eq!(a.sum_weights(), 1.0);
+        assert_eq!(a.sum_weights_sq(), 1.0);
+        assert_eq!(a.population_variance(), 0.0);
+        //assert_eq!(a.error(), 0.0);
+    }
+
+    #[test]
+    fn simple() {
+        let a: WeightedAverage = (1..6).map(|x| (f64::from(x), 1.0)).collect();
+        assert_eq!(a.mean(), 3.0);
+        assert_eq!(a.sum_weights(), 5.0);
+        assert_eq!(a.sample_variance(), 2.5);
+        //assert_almost_eq!(a.error(), f64::sqrt(0.5), 1e-16);
+    }
+}