## 2 Literature Review
### Sales Forecasting Based on Economic Indicators  
Economic indicators are a collection of aggregate factors [11; 13; 33] that can denote a country's economic conditions. Depending on the temporal causalities, economic indicators can be leading, coincident, or lagging signals [34]. A leading indicator is an economic factor that changes before the economy begins to grow or decline. Conversely, a lagging indicator is a measure that moves after a change in the economy has already occured. In contrast, coincident indicators concurrently reflect the economic condition of a country. Based on economic indicators, economists can help a country predict future conditions, and flash  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**References** & **Economic** & **Market** & **Performance** & **Methods** \\  & **Indicators** & **Analysis** & **Assessment** & **Lasso, MARS, SVR, DNN, LVM, DEA** \\ \hline This research & * & * & * & Lasso, MARS, SVR, DNN, LVM, DEA** \\ [PERSON] and [PERSON] [20] & & & & ARIMA, BPN** \\ [PERSON] and [PERSON] [27] & & & * & DEA** \\ [PERSON] et al. [21] & & & & Extreme learning \\ [PERSON] [5] & & & * & DEA** \\ [PERSON] [17] & & & & ARIMA, BPN, HWS** \\ [PERSON] et al. [19] & * & & & LVM \\ [PERSON] and [PERSON] [26] & & & & ARIMA, BPN** \\ [PERSON] et al. [28] & & & * & DEA** \\ [PERSON] et al. [12] & & & & ARIMA, wavelet, ANN** \\ [PERSON] et al. [18] & * & & LVM \\ [PERSON] et al. [22] & & & & ARIMA, BPN** \\ [PERSON] and [PERSON] [29] & & & & MARS, BPN, SVR \\ [PERSON] and [PERSON] [6] & & & Bass, Logistic, Gompertz \\ [PERSON] et al. [24] & & * & LVM \\ [PERSON] et al. [23] & & * & LVM, ARIMA \\ [PERSON] and [PERSON] [30] & & * & DEA \\ [PERSON] et al. [9] & & & Lasso, ARIMA, HWS, ES, MLR, SR \\ [PERSON] et al. [31] & & & DNN, RNN, GB \\ [PERSON] et al. [3] & & & DEA \\ [PERSON] and [PERSON] [32] & & ARIMA, BPN, ES, MLR, RF, GB, SVR \\ [PERSON] et al. [2] & & Demand distribution, QR, QRF, RF \\ \hline \hline \end{tabular} ARIMA: autoregressive integrated moving average, BPN: back propagation neural network, DEA: data envelopment analysis, DNN: deep neural network, ES: exponential smoothing, (\(\mathcal{N}\)GB: (extreme) gradient boosting, HWS: Holt-Winters smoothing, Lasso: least absolute shrinkage and selection operator, LVM: Lotka-Volterra model, MARS: multivariate adaptive regression splines, MLR: multiple linear regression, QR: quantile regression forest, RF: random forest, RT: regression tree, SVR: support vector regression, VAR: vector autoregression. * means one of the three modules (economic indicators, market analysis, performance assessment) has been addressed.  
\end{table}
Table 1: Overall comparison between this research and past studies.  
green, red, or yellow lights to alert the government, firms, consumers, and even investors regarding future changes to the economic condition.  
In practice, leading indicators help practitioners and policymers predict significant changes in the economy, while lagging indicators are used to confirm increasing or declining patterns and changes in trends [10; 29; 35]. Coincident indicators are very powerful because there are no delays between the predictors and the outcome. Regardless of whether leading, lagging, or coincident indicators are used, they must be systematically identified to recognize significant predictors. Since this study aims at the prediction of aggregate sales for retail firms, associated economic indicators, such as CPI (consumer price index), CCI (consumer confidence index), PCE (personal consumption expenditure), non-manufacturing purchase index (NMI), producer price index (PPI), industry production index (IPT), purchase manager index (PMI), regular wage, unemployment rate, oil price, and exchange rate are adopted as potential predictors in sales forecasting.