advanced29 min studyLesson 970 in Physics

Reproducible figures, tables and data provenance

Preserve the numerical and procedural information needed to reconstruct a physical result, including units, normalization and correlations.

On this page

What you’ll understand

  • Connect plotted densities to counts, efficiencies and exposure.
  • Propagate covariance through normalization and unit changes.
  • Describe a reproducible record without equating reproducibility with correctness.

A figure communicates a physical conclusion, but its pixels rarely contain enough information to reproduce the inference. Bin edges, normalization, covariance, calibration and analysis versions all affect what the points mean. Reproducible reporting preserves the path from observations to derived quantities and the assumptions made along that path. This final lesson treats a two-bin hypothetical cross-section measurement and a unit conversion. The calculations show how apparently cosmetic presentation decisions can change numerical meaning when their metadata or covariance transformations are lost.

State exactly what every plotted number represents

A bin count, an efficiency-corrected yield, an integrated cross section and a differential cross section are distinct quantities. Divide a selected count by efficiency and integrated luminosity to estimate the bin's integrated cross section under a simple no-migration model. Divide once more by bin width to obtain an average density within that bin. Unequal widths make the distinction visible: a taller count bar need not imply a proportionally taller density. A plotted bin center also requires a convention; it does not replace the stored edges.

Record whether values are before or after background subtraction, acceptance correction and unfolding. A detector response can mix truth bins, so separate binwise efficiency corrections are not a universal unfolding method. Store statistical and systematic components with enough information to preserve their correlations, along with the ordering of entries. A list of total error bars cannot reconstruct a likelihood for arbitrary theory comparisons. If asymmetric uncertainties are reported, explain whether they are interval endpoints, likelihood changes or another convention.

di=NiL ϵi Δi,Vijstat=δijNi(LϵiΔi)2,Vijnorm=fL2didjd_i=\frac{N_i}{\mathcal L\,\epsilon_i\,\Delta_i},\qquad V^{\mathrm{stat}}_{ij}=\delta_{ij}\frac{N_i}{(\mathcal L\epsilon_i\Delta_i)^2},\qquad V^{\mathrm{norm}}_{ij}=f_{\mathcal L}^2d_i d_j
The first two formulas assume independent Poisson counts, known efficiency and no background or bin migration; statistical variances use plug-in counts. The last is a first-order covariance for a shared small relative luminosity uncertainty.↔ Scroll wide equations horizontally.

Transform covariance with the values

For a linear conversion z equals A times y, covariance transforms with A on the left and its transpose on the right. Unit conversion therefore squares the numerical scale factor for variances and applies both factors to each covariance. The same rule propagates bin densities into an integrated cross section using bin widths as weights. Forgetting the off-diagonal terms can incorrectly make a common normalization uncertainty shrink when bins are added. Rounding should be delayed until after these operations.

For nonlinear transformations, a Jacobian gives a first-order approximation whose adequacy depends on uncertainty size and curvature. Exact distribution propagation or simulation may be needed near boundaries or for strongly asymmetric ratios. Retain enough numerical precision in machine-readable tables to reproduce the displayed result and avoid covariance matrices made non-positive by coarse rounding. Label whether an uncertainty is a standard deviation, a confidence interval or posterior content, and give the level and construction for intervals.

z=Ay,Vz=AVyAT,S=∑iΔidi,Var⁡(S)=∑i,jΔiVijΔjz=Ay,\qquad V_z=AV_yA^T,\qquad S=\sum_i\Delta_i d_i,\qquad \operatorname{Var}(S)=\sum_{i,j}\Delta_i V_{ij}\Delta_j
Linear covariance propagation is exact when A is fixed. The weights for the integrated quantity are the bin widths, so its units differ from those of a differential density.↔ Scroll wide equations horizontally.

Preserve a checkable chain of evidence

A useful analysis record links raw or accessible derived inputs to calibration versions, event-selection rules, transformations, fitted models and final numerical artifacts. It includes the source version, relevant configuration, software environment and random seeds when stochastic computations are used. Seeds alone do not guarantee identical execution across changed algorithms or numerical libraries. A content hash can identify the exact input bytes but cannot establish their physical correctness, provenance or permission for use without associated metadata.

Store tables in a documented machine-readable form and link them to figures, descriptions, units and immutable versions. HEPData provides a physics example of structured tables, auxiliary resources and versioned identifiers; its data-release practices illustrate why a citation should identify a particular record version. Independent reproduction should compare physical quantities and tolerances, not merely rerun one plotting command. An exactly reproduced mistake remains a mistake, so residual checks, calibration validation and independent arithmetic belong beside the provenance record rather than being replaced by it.

Two bins with a common luminosity uncertainty

Use counts 10 and 20, efficiencies 0.5 and 0.25, bin widths 1 and 2 GeV, and integrated luminosity 10 pb−110\,\mathrm{pb^{-1}}. Assume independent Poisson counts, no background or migration, and a shared 10% luminosity uncertainty treated to first order. Find the density estimates and total covariance.

  1. The denominators luminosity times efficiency times width are both numerically five in reciprocal-pb times GeV. The density estimates are therefore 2 and 4 pb GeV−14\,\mathrm{pb\,GeV^{-1}}, while the integrated bin cross sections are two and eight pb.

  2. The plug-in statistical variances of the densities are 10/25=0.410/25=0.4 and 20/25=0.820/25=0.8 in squared density units. Their statistical covariance is zero under the independent-count assumption.

  3. The shared normalization adds 0.12didj0.1^2 d_i d_j, giving diagonal additions 0.04 and 0.16 and off-diagonal entries 0.08. The total covariance has rows (0.44, 0.08) and (0.08, 0.96), in squared density units.

The result

The density vector is (2, 4) pb GeV⁻¹ and its covariance rows are (0.44, 0.08) and (0.08, 0.96) in pb² GeV⁻².

Explore the model

Interactive lab

Interactive lab · function

Area and height are distinct reported quantities

Reset and set BOTH Coefficient a = 1 and Position x = 1; predict y = 1 and signed area 1/3. For a fresh comparison, Reset and set a = 1 and x = 2; predict y = 4 and signed area 8/3.

Area and height are distinct reported quantitiesThis lab uses only y = ax². Playback traces the curve within the fixed integration interval from 0 to x; it does not estimate numerical error or change the chosen point. Current readings: y at chosen x: 0.608; Tangent slope · 2ax: 1.35; Signed area · ax³/3: 0.182. All parameters are adjustable with the labeled controls below.−2.4−1.201.22.4−9−4.504.59xy(x, ax²)
y = ax²TangentSigned integral
y at chosen x
0.608
Tangent slope · 2ax
1.35
Signed area · ax³/3
0.182
t = 0 s

This lab uses only y = ax². Playback traces the curve within the fixed integration interval from 0 to x; it does not estimate numerical error or change the chosen point. Parameter changes pause playback and recompute the model at the current elapsed time. Reset restores both parameters and returns time to zero. Playback starts paused. Adjust either parameter, then choose Play or Step.

Look forThe instrument integrates the fixed quadratic ax² from zero to x. It illustrates that endpoint height and integrated value differ; it does not bin data, normalize cross sections, unfold detector response or preserve data provenance.

Check your understanding

When converting all numerical lengths from meters to centimeters, how must their covariance matrix change?

Try it yourself

An archived length vector is (1,2) m with covariance rows (0.04,0.01) and (0.01,0.09) in square meters. Convert the vector and covariance to centimeters, then compute the variance of the difference of the two lengths.

Take a moment to make a prediction or work it out on paper. Open a hint when you need one.

Give me a hint

Apply the same factor 100 to each length and one factor for each index of covariance.

Compare with the solution

The vector becomes (100,200) cm. Multiplying every covariance entry by 10,000 gives rows (400,100) and (100,900) in square centimeters. The difference is minus one hundred cm and its variance is 400+900−2(100)=1100 cm2400+900-2(100)=1100\,\mathrm{cm^2}. The standard deviation is about 33.2 cm. Omitting the off-diagonal term would give thirteen hundred instead, and retaining the old numerical covariance after changing units would be a much larger inconsistency.

Take it with you

  • Figures need definitions, units, bin edges and uncertainty conventions.
  • Covariance must follow normalization, integration and unit transformations.
  • Versioned inputs and analysis records support reproduction but do not prove correctness.

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