# P² quantile estimator marker adjusting order

I have already written a few blog posts about the P² quantile estimator (which is a sequential estimator that uses $$O(1)$$ memory):

In this post, we continue improving the P² implementation so that it gives better estimations for streams with a small number of elements.

### The problem

In the previous post, I have performed the following experiment:

We enumerate different distributions (the standard uniform, the standard normal), different sample sizes (6, 7, 8), and different quantile probabilities (0.05, 0.1, 0.2, 0.8, 0.9, 0.95). For each combination of the input parameters, we perform 10000 simulations of the sample experiment: generate a random sample of the given size from the given distribution and estimate the given quantile using the classic initialization strategy and the new adaptive initialization strategy. As a baseline, we use the traditional Hyndman-Fan Type 7 quantile estimator. The initialization strategy that gives a better estimation (compared to the baseline) is the “winner” of the corresponding experiment. For each combination of the input parameters, we calculate the percentage of wins for each strategy.

Here is the source code of this simulation:

var random = new Random(1729);

var distributions = new IContinuousDistribution[]
{
new UniformDistribution(0, 1),
new NormalDistribution(0, 1)
};

foreach (var distribution in distributions)
foreach (int n in new[] { 6, 7, 8 })
foreach (var probability in new Probability[] { 0.05, 0.1, 0.2, 0.8, 0.9, 0.95 })
{
var randomGenerator = distribution.Random(random);
const int totalIterations = 10_000;
int classicIsWinner = 0;
for (int iteration = 0; iteration < totalIterations; iteration++)
{
var p2ClassicEstimator = new P2QuantileEstimator(probability, P2QuantileEstimator.InitializationStrategy.Classic);
var values = new List<double>();
for (int i = 0; i < n; i++)
{
double x = randomGenerator.Next();
}

double simpleEstimation = SimpleQuantileEstimator.Instance.GetQuantile(values, probability);
double p2ClassicEstimation = p2ClassicEstimator.GetQuantile();
if (Math.Abs(p2ClassicEstimation - simpleEstimation) < Math.Abs(p2AdaptiveEstimation - simpleEstimation))
classicIsWinner++;
}

int adaptiveIsWinner = totalIterations - classicIsWinner;

string title =  distribution.GetType().Name.Replace("Distribution", "").PadRight(7) + " " +
"P" + (probability * 100).ToString().PadRight(2) + " " +
"N" + n;
Console.WriteLine(\$"{title,-15}: {classicIsWinner / 100.0,6:N2}% {(adaptiveIsWinner / 100.0),6:N2}%");
}


I got the following results for the Uniform and the Normal distributions:

                 Classic Adaptive
Uniform P5  N6 :   1.31%  98.69%
Uniform P10 N6 :   2.62%  97.38%
Uniform P20 N6 :  11.44%  88.56%
Uniform P80 N6 :   0.97%  99.03%
Uniform P90 N6 :   0.00% 100.00%
Uniform P95 N6 :   0.00% 100.00%

Uniform P5  N7 :   4.00%  96.00%
Uniform P10 N7 :  14.68%  85.32%
Uniform P20 N7 :  24.52%  75.48%
Uniform P80 N7 :  10.41%  89.59%
Uniform P90 N7 :  10.31%  89.69%
Uniform P95 N7 :   1.14%  98.86%

Uniform P5  N8 :   7.50%  92.50%
Uniform P10 N8 :  22.87%  77.13%
Uniform P20 N8 :  35.12%  64.88%
Uniform P80 N8 :  24.98%  75.02%
Uniform P90 N8 :  17.81%  82.19%
Uniform P95 N8 :   3.94%  96.06%

Normal  P5  N6 :   1.73%  98.27%
Normal  P10 N6 :   3.80%  96.20%
Normal  P20 N6 :  13.93%  86.07%
Normal  P80 N6 :   1.55%  98.45%
Normal  P90 N6 :   0.00% 100.00%
Normal  P95 N6 :   0.00% 100.00%

Normal  P5  N7 :   5.86%  94.14%
Normal  P10 N7 :  21.15%  78.85%
Normal  P20 N7 :  27.72%  72.28%
Normal  P80 N7 :  12.34%  87.66%
Normal  P90 N7 :  14.50%  85.50%
Normal  P95 N7 :   1.54%  98.46%

Normal  P5  N8 :   9.18%  90.82%
Normal  P10 N8 :  32.63%  67.37%
Normal  P20 N8 :  38.88%  61.12%
Normal  P80 N8 :  28.81%  71.19%
Normal  P90 N8 :  25.45%  74.55%
Normal  P95 N8 :   4.92%  95.08%


Since both the Normal and the Uniform distributions are symmetric, we could expect symmetric results. However, the result table is asymmetric: the obtained numbers for P5/P10/P20 don’t match the corresponding numbers for P80/P90/P95.

### The solution

The final stage of the P² quantile estimator suggest adjusting non-extreme marker heights ($$q_i$$) and positions ($$n_i$$) for $$i \in \{ 1, 2, 3\}$$ (see the algorithm description and the original paper [Jain1985] for details):

for (i = 1; i <= 3; i++)
{
d = nꞌ[i] - n[i]
if (d >=  1 && n[i + 1] - n[i] >  1 ||
d <= -1 && n[i - 1] - n[i] < -1)
{
d = sign(d)
qꞌ = Parabolic(i, d)
if (!(q[i - 1] < qꞌ && qꞌ < q[i + 1]))
qꞌ = Linear(i, d)
q[i] = qꞌ
n[i] += d
}
}


The core equation of the algorithm is a piecewise-parabolic prediction (P²) formula that adjusts marker heights for each observation:

$q'_i = q_i + \dfrac{d}{n_{i+1}-n_{i-1}} \cdot \Bigg( (n_i-n_{i-1}+d)\dfrac{q_{i+1}-q_i}{n_{i+1}-n_i} + (n_{i+1}-n_i-d)\dfrac{q_i-q_{i-1}}{n_i-n_{i-1}} \Bigg).$

Once we calculated $$q'_i$$, we should check that $$q_{i-1} < q'_i < q_{i+1}$$. If this condition is false, we should ignore the parabolic prediction and use the linear prediction instead:

$q'_i = q_i + d \dfrac{q_{i+d}-q_i}{n_{i+d}-n_{i}}.$

The problem arises when the number of elements in a stream is small and we use an adjusted initialization strategy. Since we could have collisions across $$\{ n_i \}$$, the order of adjusting is important. Currently, it’s optimal only for higher quantiles ($$p > 0.5$$), but not for lower quantiles ($$p < 0.5$$). Let’s extract the adjusting logic from the above snippet to named method:

for (i = 1; i <= 3; i++)


Now it’s easy to introduce an adaptive adjusting order depending on the value of $$p$$:

if (p >= 0.5)
{
for (int i = 1; i <= 3; i++)
}
else
{
for (int i = 3; i >= 1; i--)
}


if we run the simulation from the beginning of the post, we get the following result:

                 Classic Adaptive
Uniform P5  N6 :   0.00% 100.00%
Uniform P10 N6 :   0.00% 100.00%
Uniform P20 N6 :   0.84%  99.16%
Uniform P80 N6 :   0.97%  99.03%
Uniform P90 N6 :   0.00% 100.00%
Uniform P95 N6 :   0.00% 100.00%

Uniform P5  N7 :   1.19%  98.81%
Uniform P10 N7 :   9.47%  90.53%
Uniform P20 N7 :  10.77%  89.23%
Uniform P80 N7 :  10.41%  89.59%
Uniform P90 N7 :  10.31%  89.69%
Uniform P95 N7 :   1.14%  98.86%

Uniform P5  N8 :   3.91%  96.09%
Uniform P10 N8 :  17.48%  82.52%
Uniform P20 N8 :  25.13%  74.87%
Uniform P80 N8 :  24.98%  75.02%
Uniform P90 N8 :  17.81%  82.19%
Uniform P95 N8 :   3.94%  96.06%

Normal  P5  N6 :   0.00% 100.00%
Normal  P10 N6 :   0.00% 100.00%
Normal  P20 N6 :   1.63%  98.37%
Normal  P80 N6 :   1.55%  98.45%
Normal  P90 N6 :   0.00% 100.00%
Normal  P95 N6 :   0.00% 100.00%

Normal  P5  N7 :   1.81%  98.19%
Normal  P10 N7 :  13.87%  86.13%
Normal  P20 N7 :  12.85%  87.15%
Normal  P80 N7 :  12.34%  87.66%
Normal  P90 N7 :  14.50%  85.50%
Normal  P95 N7 :   1.54%  98.46%

Normal  P5  N8 :   4.84%  95.16%
Normal  P10 N8 :  25.05%  74.95%
Normal  P20 N8 :  28.43%  71.57%
Normal  P80 N8 :  28.81%  71.19%
Normal  P90 N8 :  25.45%  74.55%
Normal  P95 N8 :   4.92%  95.08%


Now it looks quite symmetric. The problem is solved!

### The updated reference implementation

public class P2QuantileEstimator
{
private readonly int[] n = new int;
private readonly double[] ns = new double;
private readonly double[] q = new double;

public int Count { get; private set; }

public enum InitializationStrategy
{
Classic,
}

public P2QuantileEstimator(double probability,
InitializationStrategy strategy = InitializationStrategy.Classic)
{
p = probability;
this.strategy = strategy;
}

{
if (Count < 5)
{
q[Count++] = value;
if (Count == 5)
{
Array.Sort(q);

for (int i = 0; i < 5; i++)
n[i] = i;

{
Array.Copy(q, ns, 5);
n = (int)Math.Round(2 * p);
n = (int)Math.Round(4 * p);
n = (int)Math.Round(2 + 2 * p);
q = ns[n];
q = ns[n];
q = ns[n];
}

ns = 0;
ns = 2 * p;
ns = 4 * p;
ns = 2 + 2 * p;
ns = 4;
}

return;
}

int k;
if (value < q)
{
q = value;
k = 0;
}
else if (value < q)
k = 0;
else if (value < q)
k = 1;
else if (value < q)
k = 2;
else if (value < q)
k = 3;
else
{
q = value;
k = 3;
}

for (int i = k + 1; i < 5; i++)
n[i]++;
ns = Count * p / 2;
ns = Count * p;
ns = Count * (1 + p) / 2;
ns = Count;

if (p >= 0.5)
{
for (int i = 1; i <= 3; i++)
}
else
{
for (int i = 3; i >= 1; i--)
}

Count++;
}

{
double d = ns[i] - n[i];
if (d >= 1 && n[i + 1] - n[i] > 1 || d <= -1 && n[i - 1] - n[i] < -1)
{
int dInt = Math.Sign(d);
double qs = Parabolic(i, dInt);
if (q[i - 1] < qs && qs < q[i + 1])
q[i] = qs;
else
q[i] = Linear(i, dInt);
n[i] += dInt;
}
}

private double Parabolic(int i, double d)
{
return q[i] + d / (n[i + 1] - n[i - 1]) * (
(n[i] - n[i - 1] + d) * (q[i + 1] - q[i]) / (n[i + 1] - n[i]) +
(n[i + 1] - n[i] - d) * (q[i] - q[i - 1]) / (n[i] - n[i - 1])
);
}

private double Linear(int i, int d)
{
return q[i] + d * (q[i + d] - q[i]) / (n[i + d] - n[i]);
}

public double GetQuantile()
{
if (Count == 0)
throw new InvalidOperationException("Sequence contains no elements");
if (Count <= 5)
{
Array.Sort(q, 0, Count);
int index = (int)Math.Round((Count - 1) * p);
return q[index];
}

return q;
}

public void Clear()
{
Count = 0;
}
}

• [Jain1985]
Jain, Raj, and Imrich Chlamtac. “The P² algorithm for dynamic calculation of quantiles and histograms without storing observations.” Communications of the ACM 28, no. 10 (1985): 1076-1085.
https://doi.org/10.1145/4372.4378