def getUSCensusAgeDist():
    """
    Get US Census Age Distribution
    """
    t_num = _INDVCMF_DATA['USCensus2010population']

    list_AgeCensus = t_num[0]
    freq_AgeCensus = np.round(
        t_num[1] / 1000
    )  # Reduce # of populations to manageable number, this doesn't change probability

    # Remove age < 10 and 70 < age:
    freq_AgeCensus[:10] = 0
    freq_AgeCensus[71:] = 0

    list_Age = []
    for k in range(len(list_AgeCensus)):
        list_Age = np.hstack(
            (list_Age, np.repeat(list_AgeCensus[k], freq_AgeCensus[k])))

    return list_Age
def genMonteCarloObs(n_obs=1,
                     fieldsize=10,
                     list_Age=[32],
                     out='LMS',
                     wl=None,
                     allow_negative_values=False):
    """
    Monte-Carlo generation of individual observer cone fundamentals.
    
    Args: 
        :n_obs: 
            | 1, optional
            | Number of observer CMFs to generate.
        :list_Age:
            | list of observer ages or str, optional
            | Defaults to 32 (cfr. CIE2006 CMFs)
            | If 'us_census': use US population census of 2010 
              to generate list_Age.
        :fieldsize: 
            | fieldsize in degrees (between 2° and 10°), optional
            | Defaults to 10°.
        :out: 
            | 'LMS' or str, optional
            | Determines output.
        :wl: 
            | None, optional
            | Interpolation/extraplation of :LMS: output to specified wavelengths.
            | None: output original _WL = np.array([390,780,5])
        :allow_negative_values: 
            | False, optional
            | Cone fundamentals or color matching functions 
            |   should not have negative values.
            |     If False: X[X<0] = 0.
    
    Returns:
        :returns: 
            | LMS [,var_age, vAll] 
            |   - LMS: ndarray with population LMS functions.
            |   - var_age: ndarray with population observer ages.
            |   - vAll: dict with population physiological factors (see .keys()) 
            
    References:
         1. `Asano Y, Fairchild MD, and Blondé L (2016). 
         Individual Colorimetric Observer Model. 
         PLoS One 11, 1–19. 
         <http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0145671>`_
         
         2. `Asano Y, Fairchild MD, Blondé L, and Morvan P (2016). 
         Color matching experiment for highlighting interobserver variability. 
         Color Res. Appl. 41, 530–539. 
         <https://onlinelibrary.wiley.com/doi/abs/10.1002/col.21975>`_
         
         3. `CIE, and CIE (2006). 
         Fundamental Chromaticity Diagram with Physiological Axes - Part I 
         (Vienna: CIE). 
         <http://www.cie.co.at/publications/fundamental-chromaticity-diagram-physiological-axes-part-1>`_ 
         
         4. `Asano's Individual Colorimetric Observer Model 
         <https://www.rit.edu/cos/colorscience/re_AsanoObserverFunctions.php>`_
    """

    # Scale down StdDev by scalars optimized using Asano's 75 observers
    # collected in Germany:
    stdDevAllParam = _INDVCMF_STD_DEV_ALL_PARAM.copy()
    scale_factors = [0.98, 0.98, 0.5, 0.5, 0.5, 0.5, 0.5, 0.5]
    scale_factors = dict(zip(list(stdDevAllParam.keys()), scale_factors))
    stdDevAllParam = {
        k: v * scale_factors[k]
        for (k, v) in stdDevAllParam.items()
    }

    # Get Normally-distributed Physiological Factors:
    vAll = getMonteCarloParam(n_obs=n_obs)

    if list_Age is 'us_census':
        list_Age = getUSCensusAgeDist()

    # Generate Random Ages with the same probability density distribution
    # as color matching experiment:
    sz_interval = 1
    list_AgeRound = np.round(np.array(list_Age) / sz_interval) * sz_interval
    h = math.histogram(list_AgeRound,
                       bins=np.unique(list_AgeRound),
                       bin_center=True)[0]
    p = h / h.sum()  # probability density distribution

    var_age = np.random.choice(np.unique(list_AgeRound), \
                               size = n_obs, replace = True,\
                               p = p)

    # Set requested wavelength range:
    if wl is not None:
        wl = getwlr(wl3=wl)
    else:
        wl = _WL

    LMS_All = np.nan * np.ones((3 + 1, wl.shape[0], n_obs))
    for k in range(n_obs):
        t_LMS, t_trans_lens, t_trans_macula, t_sens_photopig = cie2006cmfsEx(age = var_age[k], fieldsize = fieldsize, wl = wl,\
                                                                          var_od_lens = vAll['od_lens'][k], var_od_macula = vAll['od_macula'][k], \
                                                                          var_od_L = vAll['od_L'][k], var_od_M = vAll['od_M'][k], var_od_S = vAll['od_S'][k],\
                                                                          var_shft_L = vAll['shft_L'][k], var_shft_M = vAll['shft_M'][k], var_shft_S = vAll['shft_S'][k],\
                                                                          out = 'LMS,trans_lens,trans_macula,sens_photopig')
        LMS_All[:, :, k] = t_LMS


#        listout = out.split(',')
#        if ('trans_lens' in listout) | ('trans_macula' in listout) | ('trans_photopig' in listout):
#            trans_lens[:,k] = t_trans_lens
#            trans_macula[:,k] = t_trans_macula
#            sens_photopig[:,:,k] = t_sens_photopig

    if n_obs == 1:
        LMS_All = np.squeeze(LMS_All, axis=2)

    if ('xyz' in out.lower().split(',')):
        LMS_All = lmsb_to_xyzb(LMS_All,
                               fieldsize,
                               out='xyz',
                               allow_negative_values=allow_negative_values)
        out = out.replace('xyz', 'LMS').replace('XYZ', 'LMS')
    if ('lms' in out.lower().split(',')):
        out = out.replace('lms', 'LMS')

    if (out == 'LMS'):
        return LMS_All
    elif (out == 'LMS,var_age,vAll'):
        return LMS_All, var_age, vAll
    else:
        return eval(out)