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An Analysis of Tax Revenue Forecast Errors - WP 07/02

Publication Details

  • An Analysis of Tax Revenue Forecast Errors
  • Published: Mar 2007
  • Status: Current
  • Authors: Keene, Martin; Thomson, Peter
  • JEL Classification: C53; E17; H68
  • Hard copy: Available in HTML and PDF formats only.

An Analysis of Tax Revenue Forecast Errors

New Zealand Treasury Working Paper 07/02

March 2007

Authors: Martin Keene and Peter Thomson


The New Zealand Treasury forecasts tax revenue for the twice-yearly Economic and Fiscal Updates. The accuracy of these forecasts is important for the government’s annual budget decisions as they affect key fiscal aggregates such as the operating balance and debt levels. Good decision-making in this area is important for macroeconomic stability and sustainability, one of the Treasury’s outcomes.

Over the past six years, Treasury tax forecasts, and the macroeconomic forecasts on which they are based, have underestimated the actual outturns. This report presents an analysis of the Treasury’s tax revenue forecast errors, both in aggregate and disaggregated by individual tax type.

The analysis focuses primarily on the annual one-year-ahead Budget forecasts that are typically based on rating up past tax revenues by growth rates in related macroeconomic variables such as GDP. The objective of the analysis is to better determine the major sources of tax revenue forecast error and to identify the potential for methodological improvements.

A review of the Treasury's tax forecasting methods is given and a general class of models proposed that encompasses these methods. Adopting one of the simplest of these as a benchmark, the individual tax revenue forecast errors are first disaggregated into component errors due to forecasting the macroeconomic drivers used as a proxy for the tax base, and a component due to forecasting the tax ratio, or ratio of tax revenue to proxy tax base. The tax ratio is further disaggregated into a component error due to forecasting the tax ratio trend and random error. The latter provides a measure of the best accuracy that can be achieved using the benchmark models adopted.

Among other findings, the report shows that the main source of tax revenue underforecasting is the underforecasting of the macroeconomic variables used as tax-base proxies. The tax ratio forecasts were generally unbiased, but less precisely determined than the macroeconomic forecasts. This and other evidence indicate that better tax ratio forecasts are likely to be achieved, even with the simple benchmark model used here. The benchmark models have merit as competing models that could be investigated further alongside other simple structural time series models in a systematic evaluation using historical data.

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Table of Contents


Table of Contents

List of Tables

List of Figures

1  Background

2  The Treasury's tax forecasting process

  • 2.1  Current forecasting methods

3  Towards a model framework

  • 3.1  Alternative models

4  Forecast error decompositions

  • 4.1  Benchmark model
  • 4.2  Decomposition of total tax revenue by tax type
  • 4.3  Separating out the macroeconomic forecast errors
  • 4.4  A model-based decomposition of tax forecast errors

5  Data

  • 5.1  Data issues
twp07-02-pt1.pdf (202 KB) pp. 1–14

6  Analysis

  • 6.1  Total tax revenue decomposition
  • 6.2  Individual tax revenue decompositions
  • 6.3  Other forecast horizons

7  Conclusions

twp07-02-pt2.pdf (763 KB) pp. 15–33

Appendix: Plots for two-year-ahead forecast errors


twp07-02-pt3.pdf (680 KB) pp. 34–42

List of Tables

List of Figures


We gratefully acknowledge the assistance and support received from our colleagues within the New Zealand Treasury, particularly Bob Buckle, John Carran, Katy Henderson, Dean Hyslop, John Janssen, Daniel Lawrence, Kam Szeto, Wayne Tan, and everyone else within the Treasury who has taken an interest in this research.


This document was commissioned by the New Zealand Treasury.  However, the views, opinions, findings and conclusions or recommendations expressed in it are strictly those of the authors, do not necessarily represent and should not be reported as those of the New Zealand Treasury.  The New Zealand Treasury takes no responsibility for any errors, omissions in, or for the correctness of, the information contained in this Paper.

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