Transformations and diagnostics

Peter Ralph

https://uodsci.github.io/dsci345

An example

Let's look at how newborn (i.e., "neonate") body mass is related to adult body mass, across a bunch of mammal species (in the order Carnivora, including dogs, cats, bears, weasels, and seals). (data link)

carnivores = pd.read_csv("data/carnivora_sizes.csv").set_index("Binomial")
carnivores

	Family	NeonateBodyMass_g	AdultBodyMass_g	AgeatEyeOpening_d	WeaningAge_d
Binomial
Canis aureus	Canidae	211.82	9658.70	7.50	61.30
Canis latrans	Canidae	200.01	11989.10	11.94	43.71
Canis lupus	Canidae	412.31	31756.51	14.01	44.82
Canis mesomelas	Canidae	177.20	8247.30	NaN	34.10
Callorhinus ursinus	Otariidae	5354.80	55464.82	0.00	108.69
...	...	...	...	...	...
Vulpes lagopus	Canidae	69.17	3584.37	15.03	49.50
Vulpes velox	Canidae	39.94	2088.00	12.50	47.08
Vulpes vulpes	Canidae	100.49	4820.36	14.01	50.71
Vulpes zerda	Canidae	28.04	1317.13	16.95	65.56
Zalophus californianus	Otariidae	6347.89	137194.86	0.00	319.01

145 rows × 5 columns

plt.scatter(carnivores.AdultBodyMass_g, carnivores.NeonateBodyMass_g, s=100);
plt.xlabel("adult body mass"); plt.ylabel("neonate body mass");

But - there's some "substructure"!

colors = {f: c for f, c in zip(carnivores.Family.unique(), plt.cm.tab20.colors)}
for f in colors:
    sub = carnivores[carnivores.Family == f]
    plt.scatter(sub.AdultBodyMass_g, sub.NeonateBodyMass_g, color=colors[f], s=100, label=f)
plt.xlabel("adult body mass"); plt.ylabel("neonate body mass");
plt.legend();

First pass: a normal linear model

Let's fit a linear model and see what it's predictions are like:

from sklearn.linear_model import LinearRegression as lm
fit1 = lm().fit(carnivores[['AdultBodyMass_g']], carnivores.NeonateBodyMass_g)

for f in colors:
    sub = carnivores[carnivores.Family == f]
    plt.scatter(sub.AdultBodyMass_g, sub.NeonateBodyMass_g, color=colors[f], s=100, label=f)
pred = pd.DataFrame({ "AdultBodyMass_g": np.linspace(np.min(carnivores.AdultBodyMass_g), 1.6e6, 201) })
pred['NeonateBodyMass_g'] = fit1.predict(pred)
plt.plot(pred.AdultBodyMass_g, pred.NeonateBodyMass_g)
plt.xlabel("adult mass"); plt.ylabel("neonate mass"); plt.legend();

Heteroskedasticity?

Diagnostics

Two good generic diagnostic plots are:

1. residuals versus fitted
2. distribution of residuals (histogram or QQ plot)

What's the idea?

1. If the residuals show predictable patterns, we could improve our model,
2. and if the residual SD differs systematically, we have heteroskedasticity (and so our model isn't using all the data very well).

Patterns in the residuals

Here's data that are nonlinear but we don't know it:

X = np.array([np.linspace(-4, 4, 101)]).T
y = rng.normal(loc=3*X+X**2, scale=1)
xy_lm = lm().fit(X, y)
yhat = xy_lm.predict(X)
plt.scatter(X[:,0], y)
plt.plot(X[:,0], yhat);

And, this shows up in the fit-versus-residuals plot:

resids = y - yhat
plt.scatter(yhat, resids);
plt.axhline(0, color='red')
plt.xlabel("fitted values"); plt.ylabel("residuals");

Solution: add a quadratic term!

XX2 = np.hstack([X, X**2])
xy_lm = lm().fit(XX2, y)
yhat = xy_lm.predict(XX2)
plt.scatter(X[:,0], y); plt.plot(X[:,0], yhat); plt.xlabel("x"); plt.ylabel('y');

resids = y - yhat
plt.scatter(yhat, resids);
plt.axhline(0, color='red')
plt.xlabel("fitted values"); plt.ylabel("residuals");

Back to our data

Let's look at this in our data:

carnivores['PredictedNeonateMass'] = fit1.predict(
    carnivores[['AdultBodyMass_g']]
)
carnivores['residual'] = carnivores['NeonateBodyMass_g'] - carnivores['PredictedNeonateMass']

carnivores

	Family	NeonateBodyMass_g	AdultBodyMass_g	AgeatEyeOpening_d	WeaningAge_d	PredictedNeonateMass	residual
Binomial
Canis aureus	Canidae	211.82	9658.70	7.50	61.30	916.205446	-704.385446
Canis latrans	Canidae	200.01	11989.10	11.94	43.71	1003.630922	-803.620922
Canis lupus	Canidae	412.31	31756.51	14.01	44.82	1745.209715	-1332.899715
Canis mesomelas	Canidae	177.20	8247.30	NaN	34.10	863.256460	-686.056460
Callorhinus ursinus	Otariidae	5354.80	55464.82	0.00	108.69	2634.632251	2720.167749
...	...	...	...	...	...	...	...
Vulpes lagopus	Canidae	69.17	3584.37	15.03	49.50	688.325602	-619.155602
Vulpes velox	Canidae	39.94	2088.00	12.50	47.08	632.188948	-592.248948
Vulpes vulpes	Canidae	100.49	4820.36	14.01	50.71	734.694042	-634.204042
Vulpes zerda	Canidae	28.04	1317.13	16.95	65.56	603.269588	-575.229588
Zalophus californianus	Otariidae	6347.89	137194.86	0.00	319.01	5700.752923	647.137077

145 rows × 7 columns

No particular pattern (?), but certainly: heteroskedasticity!

plt.scatter(carnivores.PredictedNeonateMass, carnivores.residual)
plt.axhline(0, color='red');
plt.xlabel("predicted neonate body mass"); plt.ylabel("residual of neonate body mass");

Why is heteroskedasticity a problem? Above, we see that small body masses are all very close to the predicted line. Indeed they have to be! A 5g weasel can't have a 100kg baby. So: these small animals are not affecting the fit of that line very much at all. Maybe that's okay - if we're interested in a relationship that holds only for big animals, for instance - but we should dig more.

A transformation

One reason for having bad model fit is that you aren't looking at the data through the right lens.

Most commonly: if the data vary by percent changes, then using standard methods (that assume additive changes) won't do so great. But, the logarithm turns percent changes into additive changes!

Let's try it out.

for f in colors:
    sub = carnivores[carnivores.Family == f]
    plt.scatter(sub.AdultBodyMass_g, sub.NeonateBodyMass_g, color=colors[f], s=100, label=f)
plt.xscale("log"); plt.yscale("log")  # log-log axes
plt.xlabel("adult mass"); plt.ylabel("neonate mass"); plt.legend();

Okay, let's now fit a model to log(adult) and log(neonate) mass:

carnivores['logAdultBodyMass_g'] = np.log(carnivores['AdultBodyMass_g'])
carnivores['logNeonateBodyMass_g'] = np.log(carnivores['NeonateBodyMass_g'])
fit2 = lm().fit(carnivores[['logAdultBodyMass_g']], carnivores.logNeonateBodyMass_g)
carnivores

	Family	NeonateBodyMass_g	AdultBodyMass_g	AgeatEyeOpening_d	WeaningAge_d	PredictedNeonateMass	residual	logAdultBodyMass_g	logNeonateBodyMass_g
Binomial
Canis aureus	Canidae	211.82	9658.70	7.50	61.30	916.205446	-704.385446	9.175614	5.355737
Canis latrans	Canidae	200.01	11989.10	11.94	43.71	1003.630922	-803.620922	9.391753	5.298367
Canis lupus	Canidae	412.31	31756.51	14.01	44.82	1745.209715	-1332.899715	10.365853	6.021775
Canis mesomelas	Canidae	177.20	8247.30	NaN	34.10	863.256460	-686.056460	9.017641	5.177279
Callorhinus ursinus	Otariidae	5354.80	55464.82	0.00	108.69	2634.632251	2720.167749	10.923504	8.585749
...	...	...	...	...	...	...	...	...	...
Vulpes lagopus	Canidae	69.17	3584.37	15.03	49.50	688.325602	-619.155602	8.184338	4.236567
Vulpes velox	Canidae	39.94	2088.00	12.50	47.08	632.188948	-592.248948	7.643962	3.687378
Vulpes vulpes	Canidae	100.49	4820.36	14.01	50.71	734.694042	-634.204042	8.480604	4.610058
Vulpes zerda	Canidae	28.04	1317.13	16.95	65.56	603.269588	-575.229588	7.183210	3.333632
Zalophus californianus	Otariidae	6347.89	137194.86	0.00	319.01	5700.752923	647.137077	11.829158	8.755878

145 rows × 9 columns

for f in colors:
    sub = carnivores[carnivores.Family == f]
    plt.scatter(sub.logAdultBodyMass_g, sub.logNeonateBodyMass_g, color=colors[f], s=100, label=f)
pred2 = pd.DataFrame({
    "logAdultBodyMass_g": np.linspace(np.min(carnivores.logAdultBodyMass_g),
                                      np.max(carnivores.logAdultBodyMass_g), 201)
})
pred2['logNeonateBodyMass_g'] = fit2.predict(pred2)
plt.plot(np.log(pred.AdultBodyMass_g), np.log(pred.NeonateBodyMass_g), label='orig fit')
plt.plot(pred2.logAdultBodyMass_g, pred2.logNeonateBodyMass_g, label='log fit')
plt.xlabel("log adult mass"); plt.ylabel("log neonate mass"); plt.legend();

fit2.coef_, fit2.intercept_, np.exp(fit2.intercept_), np.exp(np.std(np.log(pred.AdultBodyMass_g) - np.log(pred.NeonateBodyMass_g)))

(array([1.03054634]),
 -4.039480935997145,
 0.017606609001126764,
 1.4614248379177655)

Across carnivores, babies are about 1.8% the mass of adults, at birth, mostly to within about 50% of that.

Conclusions?

1. taking logs is definitely the way to fit these data
2. each family looks to have a different relationship
3. so a better model would allow different lines for each family

slopes = {}
for f in colors:
    sub = carnivores[carnivores.Family == f]
    fit_sub = lm().fit(sub[['logAdultBodyMass_g']], sub.logNeonateBodyMass_g)
    slopes[f] = fit_sub.coef_
    plt.scatter(sub.logAdultBodyMass_g, sub.logNeonateBodyMass_g, color=colors[f], s=100, label=f)
    pred_sub = pd.DataFrame({
        "logAdultBodyMass_g": np.linspace(np.min(sub.logAdultBodyMass_g),
                                          np.max(sub.logAdultBodyMass_g), 201)
    })
    pred_sub['logNeonateBodyMass_g'] = fit_sub.predict(pred_sub)
    plt.plot(pred_sub.logAdultBodyMass_g, pred_sub.logNeonateBodyMass_g, color=colors[f])
plt.legend();

Next steps: Fit the model with the additional variables to see if maybe we don't need separate models by family any more? Perhaps AgeatEyeOpening_d and WeaningAge_d?

is_nan = np.logical_or(np.isnan(carnivores.loc[:,"AgeatEyeOpening_d"]),
                       np.isnan(carnivores.loc[:,"WeaningAge_d"]))
next_data = carnivores.loc[~is_nan,:].copy()
pred_vars = ["logAdultBodyMass_g", "AgeatEyeOpening_d", "WeaningAge_d"]
next_X = next_data.loc[:,pred_vars]
next_fit = lm().fit(next_X, next_data[['logNeonateBodyMass_g']], )
next_data["predicted"] = next_fit.predict(next_data[pred_vars])

next_fit.coef_, fit2.coef_

(array([[ 8.93884874e-01, -4.98590099e-02, -3.01133804e-05]]),
 array([1.03054634]))

for f in colors:
    sub = next_data.loc[carnivores.Family == f,:]
    if sub.shape[0] > 1:
        plt.scatter(
            sub[['logAdultBodyMass_g']],
            sub[['logNeonateBodyMass_g']],
            color=colors[f], s=100, label=f
        )
pred2['AgeatEyeOpening_d'] = np.mean(next_data['AgeatEyeOpening_d'])
pred2['WeaningAge_d'] = np.mean(next_data['WeaningAge_d'])
pred2['next_pred'] = next_fit.predict(pred2[pred_vars])
plt.plot(pred2.logAdultBodyMass_g, pred2.logNeonateBodyMass_g, label='univariate fit')
plt.plot(pred2.logAdultBodyMass_g, pred2.next_pred, label='multivariate fit, at mean values')
plt.xlabel("log adult mass"); plt.ylabel("log neonate mass"); plt.legend();

fig, (ax0, ax1) = plt.subplots(1, 2)

# observed versus predicted
for f in colors:
    sub = next_data.loc[carnivores.Family == f,:]
    if sub.shape[0] > 1:
        ax0.scatter(
            sub[['predicted']],
            sub[['logNeonateBodyMass_g']],
            color=colors[f], s=100, label=f
        )
ax0.set_xlabel("predicted log neonate body mass")
ax0.set_ylabel("observed log neonate body mass")

# residual versus predicted

for f in colors:
    sub = next_data.loc[carnivores.Family == f,:]
    if sub.shape[0] > 1:
        ax1.scatter(
            sub[['predicted']],
            sub.loc[:,'logNeonateBodyMass_g'] - sub.loc[:,'predicted'],
            color=colors[f], s=100, label=f
        )
ax1.set_xlabel("predicted log neonate body mass")
ax1.set_ylabel("residual log neonate body mass")
ax0.legend();

orig_resids = next_data.loc[:,'logNeonateBodyMass_g'] - fit2.predict(next_data[["logAdultBodyMass_g"]])
new_resids = next_data.loc[:,'logNeonateBodyMass_g'] - next_data.loc[:,'predicted']

print(f"The standard deviation of the residuals for the original, univariate model was "
      f"{np.std(orig_resids):.2f}, and the for the multivariate model with eye opening and weaning times, "
      f"it is {np.std(new_resids):.2f}."
)

The standard deviation of the residuals for the original, univariate model was 0.87, and the for the multivariate model with eye opening and weaning times, it is 0.68.