C#

WITH cte As
(
    SELECT
        ROW_NUMBER() OVER (ORDER BY SeqNo) As RN,
        (TheValue - LAG(TheValue) OVER (ORDER BY SeqNo)) / TheValue As Change
    FROM
        YourTable
)
SELECT
    Avg(Change) As AverageChange
FROM
    cte
WHERE
    RN Between 1 And 4
;

double average = values.Skip(1).Take(3)
    .Zip(values, (second, first) => (second - first) / second)
    .Average();

/// <summary>
/// <a href="https://www.codeproject.com/Messages/5430072/Re-Coding-Challenge.aspx">https://www.codeproject.com/Messages/5430072/Re-Coding-Challenge.aspx</a>
/// See Richard Deemings response
/// </summary>
/// <param name="lValues">list of decimals to get the average growth from</param>
/// <returns>list of 2 growth values basedon the last value and the average growth</returns>
private List<decimal> CalcAverageGrowth(List<decimal> lValues)
{
  // lValues = new List<decimal>() { 100, 120, 118, 126, 102 };
  List<decimal> lResult = new List<decimal>();
  decimal average = lValues.Skip(1).Take(lValues.Count - 1)
.Zip(lValues, (second, first) => (second - first) / second)
.Average();
  lResult.Add(lValues.Last() + (lValues.Last() * average));
  lResult.Add(lResult.Last() + (lResult.Last() * average));
  return lResult;
}

Mycroft Holmes wrote:

lValues.Skip(1).Take(lValues.Count - 1)

Values.Select(
  p =>
    new
    {
      avg = Values.Skip(1).Take(Values.Count - 1)
        .Where(x => Math.Abs(x) > double.Epsilon)
        .Zip(Values, (second, first) => (second - first)/second)
        .Average()
    })
  .Select(x => new {x, last = Values.Last()})
  .Select(@t => new {@t, aver = @t.last + (@t.last*@t.x.avg)})
  .Select(@t => new {value1 = @t.aver, value2 = @t.aver + (@t.aver*@t.@t.x.avg)}).First();

decimal average = lValues.Skip(1)
    .Zip(lValues, (second, first) => second == 0 ? 1 : (second - first) / second)
    .Average();

// Input:    { 100, 0, 120, 118, 126, 102 }
// % Change: { 1, 1, -0.0169491525423728813559322034, 0.0634920634920634920634920635, -0.2352941176470588235294117647 }
// Output:   0.3622497586605263574356296191

Values.Select(
  p =>
    new {avg = Values.Skip(1).Where(x => index < Values.Count).Select(x => (x - Values[index++])/x).Average()})
  .Select(x => new {x, last = Values.Last()})
  .Select(@t => new {@t, aver = @t.last + (@t.last*@t.x.avg)})
  .Select(@t => new {value1 = @t.aver, value2 = @t.aver + (@t.aver*@t.@t.x.avg)}).First();

Values.Select(
  p =>
    new
    {
      avg = Values.Skip(1).Take(Values.Count - 1)
        .Zip(Values, (second, first) => (second - first)/second)
        .Average()
    })
  .Select(x => new {x, last = Values.Last()})
  .Select(@t => new {@t, aver = @t.last + (@t.last*@t.x.avg)})
  .Select(@t => new {value1 = @t.aver, value2 = @t.aver + (@t.aver*@t.@t.x.avg)}).First();

Quote:

Hiding the existence of the setter with an interface already protects against unintentional abuse, so I would say that's enough.