Recent results in the literature indicate that artificial neural networks
(ANNs) can outperform the dynamic factor model (DFM) in terms of the accuracy
of GDP nowcasts. Compared to the DFM, the performance advantage of these highly
flexible, nonlinear estimators is particularly evident in periods of recessions
and structural breaks. From the perspective of policy-makers, however, nowcasts
are the most useful when they are conveyed with uncertainty attached to them.
While the DFM and other classical time series approaches analytically derive
the predictive (conditional) distribution for GDP growth, ANNs can only produce
point nowcasts based on their default training procedure (backpropagation). To
fill this gap, first in the literature, we adapt two different deep learning
algorithms that enable ANNs to generate density nowcasts for U.S. GDP growth:
Bayes by Backprop and Monte Carlo dropout. The accuracy of point nowcasts,
defined as the mean of the empirical predictive distribution, is evaluated
relative to a naive constant growth model for GDP and a benchmark DFM
specification. Using a 1D CNN as the underlying ANN architecture, both
algorithms outperform those benchmarks during the evaluation period (2012:Q1 --
2022:Q4). Furthermore, both algorithms are able to dynamically adjust the
location (mean), scale (variance), and shape (skew) of the empirical predictive
distribution. The results indicate that both Bayes by Backprop and Monte Carlo
dropout can effectively augment the scope and functionality of ANNs, rendering
them a fully compatible and competitive alternative for classical time series
approaches.

人工神经网络 (ANNs) 可以比动态因素模型 (DFM) 更准确地预测国内生产总值 (GDP)，尤其在经济衰退和结构性断裂期间展现出灵活性和非线性估计模型的性能优势。为了填补 ANNs 无法生成带有不确定性的预测分布的空白，我们使用贝叶斯反向传播和 Monte Carlo dropout 两种不同的深度学习算法，使得 ANNs 能够生成美国 GDP 增长的概率预测。通过使用一维卷积神经网络作为底层 ANN 架构，这两种算法在评估期 (2012 年第一季度 ——2022 年第四季度) 中超越了基准模型和经典时间序列方法，而且能够动态调整预测分布的位置 (均值)、尺度 (方差) 和形状 (偏度)。研究结果表明，贝叶斯反向传播和 Monte Carlo dropout 可以有效扩展和加强 ANNs 的范围和功能，使其成为经典时间序列方法的全兼容和竞争性替代方案。