Custo Marginal (Chato)
Autor: Fabio Kanczuk | Publicado: 2012-01-27T09:42:00-02:00

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Um blog light, já que é sexta-feira. A pergunta é: O que é
esse Custo Marginal, que serve como medida de Hiato do Produto?





Num modelo DSGE (dynamic
stochastic general equilibrium) há uma economia artificial, com famílias,
firmas, governo, etc. O problema de maximização de lucro das empresas é
dividido em duas partes. A primeira envolve uma escolha de quanto capital e mão-de-obra
alocar (obtenção da função de custo). A segunda uma escolha de preço, dada a
curva de demanda pelo produto, e alguma restrição que coloca rigidez na marra (aquela
maluquice do Calvo).





Nesse mundo, o custo marginal é o conceito relevante para
indicar pressão inflacionária. Se forem feitas algumas hipóteses, tais como
função de produção Cobb-Douglas, o custo marginal se transforma em custo
unitário do trabalho (unit labor cost).
Mais algumas hipóteses heroicas, tal como a que a economia tem de ser fechada e
sem capital (sem comércio exterior e sem investimento), e o custo unitário do
trabalho se transforma em “hiato do produto”, definido como a diferença do PIB
observado e o PIB que ocorreria se não houvesse rigidez nos preços.





Dada essa visão geral, eu gostaria de ter alguma medida direta
de custo unitário do trabalho em vez do PIB filtrado. Já tentei isso com dados
da Indústria, e ficou uma porcaria mesmo na época em que a indústria era
relevante para sentir a economia. (Minha sensação é que nos últimos anos ficou
muito difícil olhar para a economia sem incluir o varejo).





Então, o que eu faço é utilizar somente dados agregados
(PIB, investimento, consumo, etc., juros, inflação e emprego) para estimar os
parâmetros do DSGE, e obter o custo marginal. Agora fazemos isso com técnicas
bayesianas, apertando o botão do dynare.
Se você não está acostumado com isso, mas conhece VARs (vector autoregressions) estruturais, é parecido. Uma vez que os
problemas das famílias, firmas, governo, etc. são resolvidos (o que pode necessitar
iteração de Bellman), o modelo DSGE fica igual um VAR, em que os parâmetros
estão restritos por condições teóricas.