the different between MEC & MEI:  i. While  the  MEC  shown  relationship  between  the  rate  of  investment  and  the  desired  stock  of  capital,  MEI  shows  the  relationship  between  the  rate  of  investment  and  the  actual  rate of investment per year (MEC shows relationship between  rate  of  investment  and  capital  stock  while  MEI  shows  relationship between rate of investment & actual investment).  ii. Thus  MEC  is  concerned  with  a  flow.  There  will  be  important  differences  between  the  capital  stock  people  desire  and  the  capital  investment  that  takes  place.  This  is  because  there  is  physical constraining upon the construction of capital goods.  ‐ The accelerator principles:    The  accelerator  model  asserts  that  investment  spending  is  proportion  to  the  changes  in  output  and  is  not  effected  by  the  cost  of  capital.  The  basic  idea  of  accelerator  principle  that  firms  install  new  capital when they need to produce more, therefore, firms would invest if  output was expected to change, but they would not otherwise undertake  new  investment.  The  simplest  view  of  the  accelerator  start  from  the  assumption a fixed rate relating output to the amount of capital normally  require to produce it, that is fixed capital output ratio.    K*t = α Yt

  It assumes that firms always adjust their capital to their output so  that the capital stock of the previous period must be in the ratio to the  output of the previous period:                Kt‐1 = α Yt‐1

Net investment is the growth in capital stock between periods:      It = K*t – Kt‐1 = α Yt – α Yt‐1 = α (Yt – Yt‐1)  It = α ΔY

  Thus  net  investment  is  proportional  to  the  growth  of  output,  rather  than its  level.  Rising  output brings  about  positive  net  investment  and constant output brings about zero net investment and falling output  brings about negative net investment.

2# 99rabbit 看的不是很明白。可以简单的讲一下吗？谢谢了.