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Assessing the Accuracy of the Probability Distributions

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Abstract

Ways of assessing the quality of the forecast densities (reported in the form of histograms) provided by the survey respondents are described and applied to the US SPF. Forecast densities can be assessed in terms of whether they could have generated the observed data: that is, whether they differ significantly from the assumed (but unknown) actual densities which gave rise to the observed data. They can also be compared to rival density forecasts, even if they are found wanting in absolute terms. In the reported assessment of the SPF aggregate and individual densities, the benchmarks are constructed to spotlight a particular aspect of the SPF densities. That is, whether the SPF respondents are able to adequately capture the time-varying uncertainty that characterized output growth and inflation. Rather than evaluating the whole densities, specific regions of interest can be considered, and this is illustrated. Finally, some scoring rules might be better suited than others when, as here, the densities are presented in the form of histograms.

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Notes

  1. 1.

    This chapter is based on Clements, M.P. (2018), Are Macroeconomic Density Forecasts Informative?, International Journal of Forecasting, 34(2), 181–198. Material from this article is used with the permission of Elsevier.

  2. 2.

    This dates back at least to Rosenblatt (1952). See Shephard (1994), Kim et al. (1998), and Diebold et al. (1998), inter alia, for recent contributions.

  3. 3.

    We do not consider this here because surveys do not report joint densities. The US SPF, for example, reports histograms for output growth, and for inflation, separately, but not for output growth and inflation jointly.

  4. 4.

    See Bao et al. (2007) and Hall and Mitchell (2009).

  5. 5.

    We could of course write (5.5) using (5.3), but this would simply introduce unknown densities, which would need to be specified.

  6. 6.

    The standard approach would be to estimate ξ 2 by \(\hat {\xi }^{2}\) where \(\hat {\xi }^{2}=\hat {\gamma }_{0}+2\sum _{j=1}^{p}\left ( \frac {p-j}{p} \right ) \hat {\gamma }_{j}\), where \(\hat {\gamma }_{0}=\frac {1}{n} \sum _{t=1}^{n}\left ( d_{t}-\overline {d}\right ) ^{2}\), \(\hat {\gamma }_{j}= \frac {1}{n}\sum _{t=j+1}^{n}\left ( d_{t}-\overline {d}\right ) \left ( d_{t-j}- \overline {d}_{j}\right ) \), and dt ≡ dtth, \(\overline {d}= \frac {1}{n}\sum _{t=1}^{n}d_{t}\), \(\overline {d_{j}}=\frac {1}{n-j} \sum _{t=j+1}^{n}d_{t-j}\).

  7. 7.

    Winkler (1967) is the classic reference on scoring rules, and more recently (Gneiting and Raftery 2007).

  8. 8.

    This is formalized by Manzan (2016) within a Bayesian framework in which an individual updates her/his prior density as new information becomes available.

  9. 9.

    The SPF provides ‘forecasts’ of the previous quarter’s value, and these are invariably set equal to the released data, that is, \( F_{80:4}^{81:1}=Y_{80:4}^{81:1}\).

  10. 10.

    We do not need to assume normality to calculate z (and hence z ): we could simply look at the proportion of the historical errors which are less than the realization. However, when this is 0 or 1, the calculation of z is problematic as the inverse normal CDF is not defined.

  11. 11.

    When, as here, z is calculated after fitting a Gaussian distribution to the histogram, taking the inverse standard normal CDF of z to give z results in \(z^{\ast }=(y-\hat {\mu })/\hat {\sigma }\), where y is the realization and \(\hat {\mu }\) and \(\hat {\sigma }^{2}\) are the mean and variance of the fitted distribution.

  12. 12.

    In fact we calculate the standard deviation of the aggregate histogram as the average of the individuals’ standard deviations. The results of doing so are indistinguishable from taking the square root of the average of the individuals’ variances. Abel et al. (2016) calculate the ‘average variance’ by first estimating the variance of the aggregate histogram, and then subtracting the disagreement term. See also Lahiri and Sheng (2010) and Lahiri et al. (2015).

  13. 13.

    Including for the first quarter output growth forecasts, that is, notwithstanding the failure to reject the unconditional benchmark densities on the Berkowitz tests, and the rejection of those which condition on location. The Berkowitz tests reject in part because the estimated variance of z is less than 1 (\(\hat {\sigma }^{2}=0.23\) for Q1 output growth forecasts). For the truly unconditional densities, the large forecast errors which result from not conditioning on location result in more extreme values of z and an estimated variance close to 1.

  14. 14.

    We used p = 3 in the expression given in the footnote in Sect. 5.2.

  15. 15.

    The 2% target was formally adopted by the FOMC at its meeting in January 2012 and was for the price index for personal consumption expenditures. See, for example, https://www.federalreserve.gov/faqs/money_12848.htm.

  16. 16.

    Especially as professional forecasters are not renowned for their ability to predict recessions: see, for example, Rudebusch and Williams (2009).

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Clements, M.P. (2019). Assessing the Accuracy of the Probability Distributions. In: Macroeconomic Survey Expectations. Palgrave Texts in Econometrics. Palgrave Macmillan, Cham. https://doi.org/10.1007/978-3-319-97223-7_5

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