variance(): number {
    const mean = this.mean();

    return this.values.reduce((s, v) => {
      const diff = v - mean;
      return s + ((diff * diff) / (this.length - 1));
    }, 0);
  }
Return the variance of the values in the `Series`

pandas equivalent: [Series.var](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.Series.var.html)

@returns {number}

@example
const ds = new Series([1, 2, 3], {name: 'New Series'})

// Returns 1
ds.variance();