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STS544 M. Camachoa et al.
                   The  single-index  dynamic  factor  model  stated  above  can  easily  be
               represented  in  state-space  form  and  be estimated  by maximum  likelihood
               using the Kalman filter. Additionally, as shown by Camacho and Doménech
               (2019), the model can be easily extended in the following dimensions:
                   1.  Dealing with data problems and variables in levels and growth rates
                   2.  Mixed frequencies. Following the approach of treating quarterly series
                      as monthly series with missing observations, Mariano and Murasawa
                      (2003) extended the single-index dynamic factor model by including
                      quarterly GDP data and monthly indicators.
                   3.  Unbalanced data sets and missing data due to different publication
                      dates, causing the so-called ragged-edge data problem at the end of
                      the sample.
                   4.  Business-cycle  nonlinearities  estimated  using  Markov-switching
                      dynamic factor models. This allows us to obtain real-time recession
                      probabilities.
                   5.  Leading indicators,  such as  the slope of the yield curve or  financial
                      tension indexes.
                   6.  Measurement errors in GDP growth.
                   We call these extensions MICA models because they are factor Models of
               economic and financial Indicators used to monitor the Current development
               of the economic Activity.

               3.  Results
                   Although BBVA’s DFM are continuously updated in real time, we restrict
               this section to the results obtained in the analysis of the models at the time
               they were published in academic journals (Camacho and Doménech, 2012,
               Camacho  and  García-Serrador,  2014,  Camacho  and  Martínez-Martín,  2014,
               Camacho, dal Bianco, and Martínez-Martín, 2015a).
                   The implementation of the methodology described in the previous section
               requires  a  selection  of  the  appropriate  indicators  from  a  list  of  potential
               business  cycle  indicators.  We  simplify  this  process  by  choosing  them
               according  to  certain  properties:  timeliness,  high  statistical  correlation  and
               relevance  (high  loading  factor).  A  meaningful  starting  point  for  selecting
               indicators entering a factor model is the approach of Stock and Watson (1991).
               Using this baseline model, new indicators are further added when the have
               statistically significant loading factors. All monthly series are made stationary
               by  differencing  or  log-differencing,  if  needed,  so  they  appear  in  quarterly
               growth rates (QGR), in monthly growth rates (MGR), in annual growth rates
               (AGR) or in levels (L). All variables are standardized by subtracting the mean
               and dividing by the standard deviation.
                   After the selection process, the set of economic indicators with statistically
               significant loading factors includes the following variables:



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